[hotfix][streaming] Reactive sanity check for windowOperator's mergeable state.

[reswqa]

What is the purpose of the change

Reactive sanity check for windowOperator's mergeable state.

Brief change log

• Remove todo in windowOperator.
• Reactive sanity check for windowOperator's mergeable state.

Verifying this change

This change is already covered by existing tests.

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:

• f6a7895 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[wangbing505]

good