Fix nullness errors in SimpleDoFnRunner and DoFnInvoker

[kennknowles]

Just fixing up a couple files I touched in the course of my daily work. There are a number of helper commits in this PR that I want to keep separate as there's no need to roll them back, and it will make the commit history cleaner for people looking at particular files.

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Reviewers are already assigned to this PR: @m-trieu

[kennknowles]

Friendly ping - this should be a quick and easy review. Mostly just adding null checks and annotation to reduce null checks.

[kennknowles]

assign to next reviewer

[kennknowles]

assign set of reviewers

[github-actions]

Reviewers are already assigned to this PR: @m-trieu

[kennknowles]

R: @Abacn

Can I ask you to look at this? Since they are core Java SDK pieces.

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[kennknowles]

Not stale - I'll resolve the conflict but the main changes here are going to be about the same.

[Abacn]

sorry missed that. Taking a look. Since this is a pretty common code path and in the past there were occurrences checkstyle fix had introduced unintended issue, I suggest check it in after release cut (so we get time to have all tests exercised)

[kennknowles]

I can also add trigger files, but this potentially touches so much it probably just makes sense to merge and rollback if there is a problem.

[kennknowles]

Well, I wouldn't say it "touches so much". I didn't actually refactor anything, just added checks and annotations mostly this time. But what I mean is "there is so much downstream of this".

[kennknowles]

Flake in :runners:java-fn-execution:test org.apache.beam.runners.fnexecution.logging.GrpcLoggingServiceTest. testMultipleClientsFailingIsHandledGracefullyByServer

[kennknowles]

got it rebased and green

[kennknowles]

friendly ping

[Abacn]

sorry for the delay, this lgtm. Retriggering the test (in order to run the PR merging onto current HEAD)