[FLINK-29324] [Connectors/Kinesis] Fix NPE for Kinesis connector when closing

[harker2015]

What is the purpose of the change

Fix the NPE issue when calling Kinesis connector close method before subtask starts running results

Brief change log

• Check object is null or not before calling

Verifying this change

This change is already covered by added unit test case

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)

[flinkbot]

CI report:

• dc4d256 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

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

[dannycranmer]

@flinkbot run azure

[dannycranmer]

LGTM. The CI failure is unrelated to this change. I will merge later today, and trigger another CI in the meantime.

[dannycranmer]

@flinkbot run azure