Handle empty batches in Python worker counters

[goutamadwant]

Handles empty batched values in OperationCounters.update_from_batch without estimating a per-element byte size when the batch converter reports length zero.

This keeps empty batches from raising ZeroDivisionError in the worker counter path. It also leaves the mean byte counter unchanged because there are no elements to average.

Addresses #38742.

Testing

From sdks/python:

• python -m pytest -q apache_beam/runners/worker/opcounters_test.py::OperationCountersTest::test_update_empty_batch
• python -m pytest -q apache_beam/runners/worker/opcounters_test.py
• python -m pytest -q apache_beam/typehints/batch_test.py
• git diff --check

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[gemini-code-assist]

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 addresses a runtime error occurring in the Python worker's operation counters when encountering empty batches. By explicitly checking for a batch length of zero, the system avoids division by zero during byte size estimation and ensures that counters remain stable when no elements are present.

Highlights

• Error Prevention: Added a check to prevent ZeroDivisionError when processing empty batches in OperationCounters.
• Test Coverage: Introduced a new test case to verify that empty batches are handled correctly without affecting counter state.

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[gemini-code-assist]

Code Review

This pull request prevents a potential division-by-zero error in opcounters.py by returning early when processing empty batches, and adds a corresponding unit test. The reviewer suggests using math.nan instead of float('nan') in the new test for better performance and idiomatic Python.

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