[FLINK-37427] Fix NPE in StandaloneHaServices and add unit testAdd unit test case for StandaloneHaServices constructor

[lurongjiang]

What is the purpose of the change

developers receive unhelpful error messages when parameters are null.

Brief change log

Replace checkNotNull(clusterRestEndpointAddress, clusterRestEndpointAddress) with checkNotNull(clusterRestEndpointAddress, "clusterRestEndpointAddress") to ensure the exception message clearly identifies the null parameter.

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

[flinkbot]

CI report:

• 0b88647 Azure: FAILURE

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[lurongjiang]

I misunderstood the merge process. I thought that getting an approval meant I should close it, but I now understand that maintainers handle the final merge.
I've restored the branch. It contains the exact same changes as before: fixing the NPE in StandaloneHaServices and adding the corresponding unit test.
I'm truly sorry for the extra work this may have caused. Please let me know if there's anything else I need to do.