[FLINK-35595][core] MailboxProcessor#processMailsWhenDefaultActionUnavailable avoids allocating Optional

[schlosna]

What is the purpose of the change

Reduce allocations and garbage collection overhead due to Optionals on hot path from org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor#runMailboxLoop.

This was observed while investigating allocation stalls and GC pressure of a Flink streaming pipeline, there were significant allocations of Optional in JFRs from org.apache.flink.streaming.runtime.tasks.mailbox. MailboxProcessor#processMailsWhenDefaultActionUnavailable().

Brief change log

• Remove unnecessary allocation of Optionals in org.apache.flink.streaming.runtime.tasks.mailbox. MailboxProcessor#processMailsWhenDefaultActionUnavailable() method

Verifying this change

This change is already covered by existing tests org.apache.flink.streaming.runtime.tasks.mailbox.TaskMailboxProcessorTest.

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): not per record, but mailbox is performance sensitive
• 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:

• 2525946 Azure: CANCELED

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[github-actions]

This PR is being marked as stale since it has not had any activity in the last 90 days.
If you would like to keep this PR alive, please leave a comment asking for a review.
If the PR has merge conflicts, update it with the latest from the base branch.

If you are having difficulty finding a reviewer, please reach out to the
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If this PR is no longer valid or desired, please feel free to close it.
If no activity occurs in the next 30 days, it will be automatically closed.

[schlosna]

@flinkbot run azure

[github-actions]

This PR is being marked as stale since it has not had any activity in the last 90 days.
If you would like to keep this PR alive, please leave a comment asking for a review.
If the PR has merge conflicts, update it with the latest from the base branch.

If you are having difficulty finding a reviewer, please reach out to the
community, contact details can be found here: https://flink.apache.org/what-is-flink/community/

If this PR is no longer valid or desired, please feel free to close it.
If no activity occurs in the next 30 days, it will be automatically closed.