Preserve dictionary-value nulls in scalar regex operators #23963

Describe the bug

regex_match_dyn_scalar evaluates a dictionary's values once and gathers the resulting booleans with BooleanArray::take_iter. The gather preserves null dictionary keys, but it does not preserve nulls in the evaluated dictionary values.

As a result, a valid key that references a null dictionary value becomes a valid false. Negated regex operators then turn that value into true, which violates SQL three-valued logic and differs from evaluating the same input after casting it to Utf8.

To Reproduce

Construct a dictionary array whose values contain both a string and a null, and whose keys contain both valid keys and a null key. Evaluate it against a scalar pattern with:

• RegexMatch
• RegexIMatch
• RegexNotMatch
• RegexNotIMatch

Compare the result with evaluation after casting the dictionary array to Utf8. The current dictionary-specialized path loses validity for keys that reference the null dictionary value; the cast path retains it.

Expected behavior

Scalar regex evaluation on dictionary input should preserve both dictionary-key validity and dictionary-value validity. Its values and null bitmap should match the equivalent cast-to-Utf8 evaluation for all four regex operators.

Additional context

The dictionary/scalar optimization was introduced in #12768. Current main and the 54.1.0 tag still use the validity-dropping take_iter gather.

#23722 is related but distinct: it addresses dictionary input in the array-pattern regex_match_dyn path, while this report concerns null propagation in regex_match_dyn_scalar.

A downstream reproduction and focused fix are available in GreptimeTeam#22.