[Java] Add ByteStringOutputStream.isEmpty() and reduce ByteStringOutputStream.size() calls

[scwhittle]

This is cheaper than size() which would have to iterate through ByteString trees.
size() was showing up as .5% of CPU in a test pipeline.

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Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a performance improvement by introducing a new isEmpty() method to ByteStringOutputStream. This method provides a more efficient way to check if a stream is empty compared to the existing size() method, which can be computationally expensive due to iterating through ByteString trees. The changes replace size() calls with isEmpty() where appropriate, leading to reduced CPU usage in critical paths, and also include minor corrections to byte counting logic in related stream implementations.

Highlights

• New API for checking emptiness: Introduced a new isEmpty() method in ByteStringOutputStream to efficiently check if the stream contains any data without incurring the cost of calculating its total size.
• Performance Optimization: Replaced existing stream.size() == 0 or stream.size() > 0 checks with the new isEmpty() method across various components (PubsubSink, WindmillSink, BeamFnDataOutboundAggregator, DataStreams, BagUserState) to reduce CPU overhead, as size() can be expensive for ByteString trees.
• Improved Byte Counting Logic: Corrected the byte counting logic in BeamFnDataOutboundAggregator.Receiver and DataStreams.ElementDelimitedOutputStream to properly account for zero-byte encodings and ensure accurate delta calculations.
• Stream Reset Optimization: Optimized the reset() method in ByteStringOutputStream for simpler and more direct state clearing.
• Test Updates: Updated relevant unit tests to utilize the new hasBufferedOutput() method and added a test case for the isEmpty() method.

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[scwhittle]

R: @arunpandianp

[github-actions]

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