[Embedder Keyboard] Fix synthesized events causing crash

[dkwingsmt]

This PR fixes a problem where synthesized events will cause crash, a bug introduced by #29638.

This PR also changes many keyboard-related tests in embedder_unittests.cc into using nullptr as the callback. This is because dealing with callback requires special setup (see test KeyDataResponseIsCorrectlyInvoked). Setting these useless callbacks to nullptr might help reduce confusion (which I actually encountered.)

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[gspencergoog]