Ensure clipboard text is non-null (#5482)
Flutter behaves unexpectedly if you set your clipboard text to null:
1. Android and iOS will set your clipboard content to the text "null".
2. Linux and web throws
3. Windows crashes
Flutter will be updated to disallow setting the clipboard text to null: https://github.com/flutter/flutter/pull/122446
Part of https://github.com/flutter/flutter/issues/121976
