[FLINK-26596][BP-1.14][runtime][test] Adds leadership loss handling

[XComp]

This is a 1.14 backport of PR #19066

What is the purpose of the change

If the ZK connection is flaky, we might collect another
ZK connection loss. This is now handled properly.

Brief change log

• Adds null check to condition

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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): no
• 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:

• 749c4b3 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

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

[XComp]

Errors in ci run are unrelated:

• FLINK-26220
• FLINK-26621

[XComp]

rebased release-1.14 after the FLINK-26121 backport PR was merged

[XComp]

Test failure resulted due an internal error when publishing build artifacts. Unrelated to the change. No ticket created because there are plans to switch to GHA anyway