fix(plugin): check if null passed to objectSupport parser
this fixes #2095 
**Describe the bug**
`dayjs(null)` is throws error, 'Uncaught TypeError: can't access property "constructor", n is null'.
It says that error occured on dayjs/plugin/objectSupport.js
```
11:11:19.151 objectSupport.js:1 Uncaught TypeError: Cannot read properties of null (reading 'constructor')
    at M.r (objectSupport.js:1:412)
    at i.parse (objectSupport.js:1:887)
    at e.parse (preParsePostFormat.js:1:447)
    at u.parse (utc.js:1:692)
    at new M (dayjs.min.js:1:2038)
    at w (dayjs.min.js:1:1869)
    at <anonymous>:1:1
r @ objectSupport.js:1
i.parse @ objectSupport.js:1
e.parse @ preParsePostFormat.js:1
u.parse @ utc.js:1
M @ dayjs.min.js:1
w @ dayjs.min.js:1
```

**Expected behavior**
It should be return dayjs object with invalid date. (it means, `dayjs(null).isValid()` return false correctly.)

**Information**
 - Day.js Version: v1.11.6
 - OS: Windows 10
 - Browser: Firefox latest version
 - Time zone: GMT+09:00
 - Used Plugins: 
   - customParseFormat
   - duration
   - isoWeek
   - isSameOrBefore
   - objectSupport

`dayjs(null)` throws error, not return dayjs object as invalid date
diff --git a/src/plugin/objectSupport/index.js b/src/plugin/objectSupport/index.js
index 650166606..bd32358e9 100644
--- a/src/plugin/objectSupport/index.js
+++ b/src/plugin/objectSupport/index.js
@@ -1,6 +1,6 @@
 export default (o, c, dayjs) => {
   const proto = c.prototype
-  const isObject = obj => !(obj instanceof Date) && !(obj instanceof Array)
+  const isObject = obj => obj !== null && !(obj instanceof Date) && !(obj instanceof Array)
         && !proto.$utils().u(obj) && (obj.constructor.name === 'Object')
   const prettyUnit = (u) => {
     const unit = proto.$utils().p(u)

diff --git a/test/plugin/objectSupport.test.js b/test/plugin/objectSupport.test.js
index 06b10f9dc..7e20200b3 100755
--- a/test/plugin/objectSupport.test.js
+++ b/test/plugin/objectSupport.test.js
@@ -384,3 +384,11 @@ it('add decimal values of days and months', () => {
   expect(dayjs([2016, 7, 1]).add(1.6, 'years').format('YYYY-MM-DD')).toBe('2017-07-01')
   expect(dayjs([2016, 1, 1]).add(1.1, 'quarters').format('YYYY-MM-DD')).toBe('2016-04-01')
 })
+
+it('returns valid date on undefined', () => {
+  expect(dayjs().isValid()).toBe(true)
+})
+
+it('returns invalid date on null', () => {
+  expect(dayjs(null).isValid()).toBe(false)
+})

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