Fix nullness for schemas/logicaltypes

[kennknowles]

While reviewing code touching this, it seemed like having the files annotated and analysis active would be helpful.

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[kennknowles]

R: @baeminbo

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[kennknowles]

I cannot reproduce the failure in the precommit locally - I believe it is a flake. Please do review while I kick the tests repeatedly.

[kennknowles]

You could also just cherrypick the commit onto your PR (and reorder the commits however you want). That might be the simplest thing to do.

[baeminbo]

I cherry-picked the commit to https://github.com/apache/beam/pull/35150