Fix up some nullability annotations to remove unnecessary null-forgiving operations...

Fix up some nullability annotations to remove unnecessary null-forgiving operations (!) (#32186)

* Fix up some nullability annotations to remove unnecessary null-forgiving
operations (!)

* Revert change to TryGetOrAddReferenceOnSerialize since it will never
return null referenceId string.

* Remove unused using directive.

* Adding NotNull on the JsonConverter<T>.Write and MaybeNullWhen on the
OnTryRead method overrides.

* Address feedback - DisallowNull instead of NotNull and remove
comment/debug.assert.

* Add back Debug.Asserts because I like them :)