feat(expr): allow jsonb null values to be cast to scalar types as null

[howenyap]

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Requested in #23368.

• regression tests: added but excluded with --@ since postgres 15 is used for regression test job
• slt tests: added test cases to jsonb.slt.part

What's changed and what's your intention?

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note