[FLINK-35440][Table SQL / JDBC] feat(flink-connection): support isValid for tableau

[blaskovicz]

What is the purpose of the change

Allow Tableau to connect to flink sql via jdbc driver. Currently, tableau fails when FlinkConnection#isValid is called because it is not implemented. See the associated issue for a description of the exception.

Brief change log

• isValid returns true always for FlinkConnection

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:

• 7026b60 Azure: SUCCESS

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[jectpro7]

should it check the closed flag rather than just return ture?