[FLINK-39145][tests] Skip counters accumulation when state isn't initialized

[Efrat19]

What is the purpose of the change

Sink task might be cancelled before state was initialized, should be handled as a valid scenario to avoid test failures due to null state.

Brief change log

• Skip counters accumulation in VerifyingSinkBase#close when state isn't initialized to avoid test falkiness

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

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Was generative AI tooling used to co-author this PR?

no

[flinkbot]

CI report:

• 81e655b Azure: SUCCESS

Bot commands

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[Efrat19]

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@1996fanrui when you have time

[1996fanrui]

Thanks for the fix, lgtm