Optimze away WindmillWatermarkHold::clear when the cached hold is empty

[arunpandianp]

No description provided.

[arunpandianp]

R: @scwhittle

[gemini-code-assist]

Summary of Changes

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This pull request introduces a performance optimization for the WindmillWatermarkHold class by preventing redundant operations when the watermark hold is already cleared. By adding an early return check, the system avoids unnecessary processing and state modifications, improving efficiency in scenarios where clear() is called on an empty state.

Highlights

• Performance Optimization: Added a conditional check in WindmillWatermarkHold::clear to return early if the cached hold is already empty, avoiding unnecessary state updates.
• Testing: Added a new test case testWatermarkClearNoOp to verify that clearing an empty watermark hold does not result in unnecessary state persistence.

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[github-actions]

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[arunpandianp]

StreamingDataflowWorkerTest tests fail due to problems with mocks, looking into fixing them.

[scwhittle]

How about:

localAdditions = null;
if (cachedValue == null || cachedValue.isPresent()) {
  cleared = true;
  cachedValue = Optional.absent();
}

no reason to not skip sending backend clear if there were localadditions