fix(core, reporter): default exception handler may be null

[mikehardy]

Underlying Java API documents it as potentially null
https://developer.android.com/reference/java/lang/Thread#getDefaultUncaughtExceptionHandler()

Null values are handled in ReportExecutor (the only consumer of this value that I know of?):

acra/acra-core/src/main/java/org/acra/builder/ReportExecutor.kt

Line 51 in 042eddd

private val defaultExceptionHandler: Thread.UncaughtExceptionHandler?, private val processFinisher: ProcessFinisher,

Fixes a crash in robolectric contexts where it actually is null

Test Plan: I made the correct package directory structure, copied the original version of the file in to AnkiDroid's codebase that way and ran it - reproduce the failure in our robolectric tests - ran this changed version and it worked

Also ./gradlew test passed locally

[mikehardy]

Yes!! Thanks @F43nd1r - this is the last thing stopping us from finally moving off our 5.7 versions with annotation config, through 5.8 up to 5.9....2 I guess :-), with it's new config style and everything. Much appreciated!

[F43nd1r]

5.9.3 in the end, because 5.9.2 failed to publish due to Kotlin/dokka#2452

[mikehardy]

It's always something...