HttpException.response() should be marked w/ @Nullable
What kind of issue is this?

 - [x] Bug report

Code for that method in Retrofit2 2.5 is:

```
/**
   * The full HTTP response. This may be null if the exception was serialized.
   */
  public Response<?> response() {
    return response;
  }
```
retrofit/src/main/java/retrofit2/HttpException.java
retrofit/src/main/java/retrofit2/HttpException.java
diff --git a/retrofit/src/main/java/retrofit2/HttpException.java b/retrofit/src/main/java/retrofit2/HttpException.java
index c01f97dfe8..f53e76e782 100644
--- a/retrofit/src/main/java/retrofit2/HttpException.java
+++ b/retrofit/src/main/java/retrofit2/HttpException.java
@@ -15,6 +15,8 @@
  */
 package retrofit2;
 
+import javax.annotation.Nullable;
+
 import static retrofit2.Utils.checkNotNull;
 
 /** Exception for an unexpected, non-2xx HTTP response. */
@@ -48,7 +50,7 @@ public String message() {
   /**
    * The full HTTP response. This may be null if the exception was serialized.
    */
-  public Response<?> response() {
+  public @Nullable Response<?> response() {
     return response;
   }
 }
