[FLINK-33926][kubernetes]: Allow using job jars in the system classpath in native mode

[tinaselenge]

What is the purpose of the change

This PR removes the verification for pipeline.jars if the option is not set, which allows using job jars in system classpath. FLINK-29288 has resolved this already for standalone mode, however in native mode, it currently results in NullPointerException.

This PR resolves both https://issues.apache.org/jira/browse/FLINK-33926 and https://issues.apache.org/jira/browse/FLINK-36137

Verifying this change

This change added a new unit test that executes deployment without pipeline.jars and checks no exception is thrown.

I have ran mvn verify for the module and passed existing tests for kubernetes module. I've also deployed an operator with a custom Docker image built based on this branch in minikube environment. I confirmed that my custom Flink Docker image with a job jar in /opt/flink/lib can be executed successfully.

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 )

The pipeline.jars argument is already documented as an optional, but not required argument to provide. The doc says, if this option is not specified, jars in system classpath will be used. This is not true for native mode currently.

[flinkbot]

CI report:

• b8fb2923ad5c958a7af8664f536effb987aad16f Azure: SUCCESS

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[github-actions[bot]]

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[tinaselenge]

This PR still needs a review from a maintainer.