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: flutter/flutter#122446

Part of flutter/flutter#121976

RELEASE_NOTE_EXCEPTION=Null safety change that shouldn't affect functionality.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or there is a reason for not adding tests.

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