[FLINK-39821][table] Make REGEXP_REPLACE return type nullable

[raminqaf]

Issue was mentioned post-merge in this PR: #28189 (comment)

What is the purpose of the change

REGEXP_REPLACE declared its output via nullableIfArgs, so the planner inferred a NOT NULL result when all three arguments were NOT NULL. But the runtime returns null for a
non-literal regex that fails to compile: a column reference or a CONCAT(...) result is only validated at runtime, not at planning time. That let a null flow through a column the
planner believed was non-null, which is unsound.

This is a sub-task of the FLINK-39648 umbrella.

Brief change log

• Change BuiltInFunctionDefinitions.REGEXP_REPLACE output type strategy from nullableIfArgs(explicit(STRING())) to explicit(DataTypes.STRING().nullable()), matching
  REGEXP_EXTRACT, which is always-nullable for the same reason. (REGEXP keeps nullableIfArgs because it returns false, not null, on an invalid pattern.)

Verifying this change

This change added a test and can be verified as follows:

• New RegexpFunctionsITCase case with NOT NULL arguments and a non-literal invalid regex, asserting the output type stays nullable and the value is null.
• Existing RegexpFunctionsITCase, ScalarFunctionsTest, SqlExpressionTest, MultiJoinTest (plan digests unchanged), and ExpressionSerializationTest regress the function.

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: 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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Generated-by: Opus 4.8

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CI report:

• 2a2b328 Azure: SUCCESS

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[dylanhz]

Thanks for the quick fix, LGTM!

[fhueske]

Thanks for the fix!

I'll merge this 🙌