fix: fix diff bug when UTC plugin enabled
fix #1200
**Describe the bug**

1.9.4
```
dayjs('2020-11-02T02:58:28.000-08:00').diff()
-703365300
```

1.9.5 and 1.9.6
```
dayjs('2020-11-02T02:58:28.000-08:00').diff()
TypeError: Cannot read property '$u' of undefined
    at diff (/home/circleci/project/node_modules/dayjs/plugin/utc.js:1:1941)
```


**Information**
 - Day.js Version 1.9.6
 - OS: macOS 10.15.17
 - Browser chrome 85.0.4183.121
 - Time zone: GMT+3

Cannot read property '$u' of undefined
diff --git a/src/plugin/utc/index.js b/src/plugin/utc/index.js
index 5fb049793..b5b5f4e64 100644
--- a/src/plugin/utc/index.js
+++ b/src/plugin/utc/index.js
@@ -112,7 +112,7 @@ export default (option, Dayjs, dayjs) => {
   }
   const oldDiff = proto.diff
   proto.diff = function (input, units, float) {
-    if (this.$u === input.$u) {
+    if (input && this.$u === input.$u) {
       return oldDiff.call(this, input, units, float)
     }
     const localThis = this.local()

diff --git a/test/plugin/utc.test.js b/test/plugin/utc.test.js
index 7ef563cd9..d9761f9cb 100644
--- a/test/plugin/utc.test.js
+++ b/test/plugin/utc.test.js
@@ -237,6 +237,9 @@ describe('Diff', () => {
       expect(_.utc(d1).diff(_.utc(d2), 'm')).toBe(1440)
     })
   })
+  it('default diff', () => {
+    expect(dayjs().diff()).toBeDefined()
+  })
   it('local.diff(utc)', () => {
     expect(dayjs(d1).diff(dayjs.utc(d2), 'days'))
       .toBe(moment(d1).diff(moment.utc(d2), 'days'))

{"test/plugin/utc.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"test/timezone.test.js:Add Time days (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/de.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sk.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/advancedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrBefore.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/pluralGetSet.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Diff (DST)": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/keys.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/utils.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/relativeTime.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/parse.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/localizedFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/updateLocale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/constructor.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isLeapYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/query.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/localeData.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC diff in DST": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/zh.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/declarations.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/timezone.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/ru.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/dayOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC add day in DST": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/manipulate.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/cs.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/en.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/get-set.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/quarterOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/objectSupport.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/fi.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sv.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toArray.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:Utc Offset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/utc-utcOffset.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sr-cyrl.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/br.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isMoment.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isYesterday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/pl.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/toObject.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/sr.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isSameOrAfter.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/weekOfYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/comparison.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/calendar.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/hu.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isTomorrow.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/et.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/timezone.test.js:UTC and utcOffset": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/arraySupport.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isToday.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/hr.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/badMutable.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/customParseFormat.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/duration.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeek.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/minMax.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isoWeeksInYear.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/buddhistEra.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/display.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/uk.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/lt.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/locale/zh-cn.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}, "test/plugin/isBetween.test.js": {"run": "PASS", "test": "PASS", "fix": "PASS"}}
{"test/plugin/utc.test.js": {"run": "PASS", "test": "FAIL", "fix": "PASS"}}
{"passed_count": 71, "failed_count": 0, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/get-set.test.js", "test/plugin.test.js", "test/locale/de.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/isSameOrBefore.test.js", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/sr-cyrl.test.js", "test/locale/br.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/locale/keys.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/locale/sr.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/plugin/isLeapYear.test.js", "test/comparison.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/plugin/calendar.test.js", "test/locale/hu.test.js", "test/plugin/isTomorrow.test.js", "test/timezone.test.js:UTC diff in DST", "test/locale/et.test.js", "test/timezone.test.js:UTC and utcOffset", "test/plugin/arraySupport.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/locale/hr.test.js", "test/plugin/timezone.test.js", "test/locale.test.js", "test/plugin/badMutable.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/duration.test.js", "test/plugin/isoWeek.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/minMax.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/display.test.js", "test/locale/cs.test.js", "test/plugin/buddhistEra.test.js", "test/locale/uk.test.js", "test/locale/lt.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": [], "skipped_tests": []}
{"passed_count": 70, "failed_count": 1, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/plugin.test.js", "test/get-set.test.js", "test/locale/de.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/plugin/isSameOrBefore.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/sr-cyrl.test.js", "test/utils.test.js", "test/locale/br.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/locale/keys.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/locale/sr.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/comparison.test.js", "test/plugin/isLeapYear.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/plugin/isTomorrow.test.js", "test/locale/hu.test.js", "test/locale/et.test.js", "test/timezone.test.js:UTC diff in DST", "test/plugin/calendar.test.js", "test/timezone.test.js:UTC and utcOffset", "test/plugin/arraySupport.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/locale/hr.test.js", "test/plugin/timezone.test.js", "test/plugin/badMutable.test.js", "test/locale.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/isoWeek.test.js", "test/plugin/duration.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/minMax.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/locale/uk.test.js", "test/plugin/buddhistEra.test.js", "test/locale/cs.test.js", "test/display.test.js", "test/locale/lt.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": ["test/plugin/utc.test.js"], "skipped_tests": []}
{"passed_count": 71, "failed_count": 0, "skipped_count": 0, "passed_tests": ["test/timezone.test.js:Add Time days (DST)", "test/get-set.test.js", "test/plugin.test.js", "test/locale/de.test.js", "test/plugin/utc.test.js", "test/plugin/quarterOfYear.test.js", "test/plugin/objectSupport.test.js", "test/locale/fi.test.js", "test/locale/sk.test.js", "test/locale/sv.test.js", "test/plugin/advancedFormat.test.js", "test/plugin/toArray.test.js", "test/plugin/weekYear.test.js", "test/timezone.test.js", "test/plugin/isSameOrBefore.test.js", "test/timezone.test.js:Utc Offset", "test/plugin/pluralGetSet.test.js", "test/timezone.test.js:Diff (DST)", "test/plugin/utc-utcOffset.test.js", "test/locale/sr-cyrl.test.js", "test/locale/br.test.js", "test/utils.test.js", "test/plugin/relativeTime.test.js", "test/parse.test.js", "test/locale/keys.test.js", "test/plugin/isMoment.test.js", "test/plugin/isYesterday.test.js", "test/locale/pl.test.js", "test/plugin/toObject.test.js", "test/locale/sr.test.js", "test/plugin/localizedFormat.test.js", "test/plugin/updateLocale.test.js", "test/constructor.test.js", "test/plugin/weekday.test.js", "test/plugin/isSameOrAfter.test.js", "test/plugin/weekOfYear.test.js", "test/plugin/isLeapYear.test.js", "test/comparison.test.js", "test/query.test.js", "test/plugin/localeData.test.js", "test/locale/et.test.js", "test/locale/hu.test.js", "test/plugin/isTomorrow.test.js", "test/timezone.test.js:UTC diff in DST", "test/plugin/calendar.test.js", "test/timezone.test.js:UTC and utcOffset", "test/plugin/arraySupport.test.js", "test/locale/zh.test.js", "test/plugin/isToday.test.js", "test/plugin/declarations.test.js", "test/locale/hr.test.js", "test/plugin/timezone.test.js", "test/plugin/badMutable.test.js", "test/locale.test.js", "test/plugin/customParseFormat.test.js", "test/locale/ru.test.js", "test/plugin/duration.test.js", "test/plugin/isoWeek.test.js", "test/plugin/minMax.test.js", "test/plugin/dayOfYear.test.js", "test/plugin/isoWeeksInYear.test.js", "test/timezone.test.js:UTC add day in DST", "test/manipulate.test.js", "test/plugin/buddhistEra.test.js", "test/locale/cs.test.js", "test/display.test.js", "test/locale/uk.test.js", "test/locale/lt.test.js", "test/locale/zh-cn.test.js", "test/locale/en.test.js", "test/plugin/isBetween.test.js"], "failed_tests": [], "skipped_tests": []}