[FLINK-37840] [table] Row writer should honor null uncompact BigDecimal and Timestamp

[dengziming]

What is the purpose of the change

Fix NPE when writing null uncompact BigDecimal and Timestamp

Brief change log

since we support passing null compact BigDecimal and Timestamp, we should also support passing null uncompact ones.

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change added tests and can be verified as follows:

Adding more test cases to existing cases.

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): (don't know, maybe)
• 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:

• 584c6691a6e17c7e009a558bcaadbfb9e24d7a27 Azure: SUCCESS

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[dengziming]

@wuchong can you help look if this change is reasonable?