fix TypeCastException when unlinking google account

[timfreiheit]

fixes kotlin.TypeCastException: null cannot be cast to non-null type com.parse.ParseException

when trying to unlink the ParseUser from Google.

The Task returns null as the error when the task was completed successfully.
This is the same behavior as in the ParseFacebookUtils but was handled incorrectly here.

[Jawnnypoo]

Thanks for the fix!