[FLINK-23765][python] Fix the NPE in Python UDTF

[HuangXingBo]

What is the purpose of the change

This pull request will fix the NPE in Python UDTF

Brief change log

• Change the logic of emitResult in Python Table Function Operator for facing parts of results have been received

Verifying this change

This change added tests and can be verified as follows:

• Original Java UT and python IT

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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Automated Checks

Last check on commit 796d67e (Thu Aug 19 03:31:43 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

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• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• 796d67e Azure: SUCCESS

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