Refactor: Fix critical state management bug and improve null safety

[miladsade96]

Changes:

1. Correcting state management bug
2. Handling null value properly

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[miladsade96]

I have signed the google cla, but still shows me as unsigned. Is there any way to force cla/google to reload or recheck the cla status?

[cpovirk]

I clicked the link for rescanning it from https://github.com/google/guava/pull/8006/checks?check_run_id=50725139081, and now it's showing as passing.

If this PR fixes a bug, please add a test that fails without the PR and passes with it.

[miladsade96]

@cpovirk
I refactored the code but the behavior is the same as before.
Is it necessary to add more tests to the test class?

[cpovirk]

Ah, OK, I was going off the PR description.

It looks to me like the PR introduces a bug: It no longer allows computeNext() to return a null to be returned to users. Instead, it treats any null as the end of the iteration. (The docs note: "This class supports iterators that include null elements.".) From what I've seen so far, I don't think we have tests for that case. We probably should.

[chaoren]

critical state management bug

Could you please describe what the bug is in the commit message and PR description?

Handling null value properly

Please describe where in the existing code was null not properly handled.

I refactored the code but the behavior is the same as before.

If the behavior is the same as before, then where is the bug?

There's a lot of shuffling around of existing code in the commit and it's not properly formatted, so it's difficult to tell what actually changed from just the diff.