[FLINK-35661][table] Fix MiniBatchGroupAggFunction silently dropping records (backport #27505)

[nateab]

Backport of #27505 to release-1.20.

What is the purpose of the change

This pull request fixes a bug in MiniBatchGroupAggFunction.finishBundle() where records were being silently dropped when a mini-batch bundle contained a key with only retraction messages and no existing accumulator state.

The root cause was using return instead of continue when inputRows became empty after filtering out leading retraction messages for a key with no state. This caused the method to exit entirely, abandoning processing of all remaining keys in the bundle.

Brief change log

• Changed return; to continue; in MiniBatchGroupAggFunction.finishBundle() so processing continues to the next key instead of exiting the entire method
• Added unit test MiniBatchGroupAggFunctionTest that directly verifies the fix by simulating a bundle with multiple keys where the first key has only retractions

Verifying this change

This change added tests and can be verified as follows:

• Added MiniBatchGroupAggFunctionTest.testFinishBundleContinuesAfterEmptyInputRows() which creates a mock bundle with three keys where the first key has only a DELETE message (no existing state). The test verifies that:
  • Without the fix: 0 outputs are produced (all subsequent keys dropped)
  • With the fix: 2 outputs are produced (keys "bbb" and "ccc" processed correctly)

Backport notes:

• Cherry-picked cleanly from b81b124 (master). The source file is byte-identical between master pre-fix and release-1.20.
• Test passes locally on release-1.20: Tests run: 1, Failures: 0, Errors: 0.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): yes (mini-batch aggregation path, but change is minimal - only control flow fix)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• eb3440c Azure: PENDING

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