fix: Propagate NULLs in regexp_count, regexp_instr

[neilconway]

Which issue does this PR close?

• Closes Inconsistent NULL handling in regexp_count, regexp_instr #24152

Rationale for this change

regexp_count and regexp_instr should follow the behavior of these functions in PostgreSQL and return NULL when any of their arguments is NULL. The previous behavior was inconsistent: regexp_instr returned NULL for a NULL string or pattern, but otherwise NULLs were treated as a default value, returned a match count of 0, or raised an error, depending on the context.

What changes are included in this PR?

• Make NULL handling of regexp_count and regexp_instr consistent and match PostgreSQL
• Update SLTs or add new SLTs / unit tests as necessary

Are these changes tested?

Yes, new tests added and/or existing tests updated when necessary.

Are there any user-facing changes?

Yes: the behavior of these UDFs has changed when called with NULL arguments.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.48529% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.00%. Comparing base (d5bd10d) to head (7e535f1).

Files with missing lines	Patch %	Lines
datafusion/functions/src/regex/regexpcount.rs	91.41%	13 Missing and 1 partial ⚠️
datafusion/functions/src/regex/regexpinstr.rs	99.08%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24239      +/-   ##
==========================================
+ Coverage   80.98%   81.00%   +0.01%     
==========================================
  Files        1106     1106              
  Lines      383495   383697     +202     
  Branches   383495   383697     +202     
==========================================
+ Hits       310590   310810     +220     
+ Misses      54572    54551      -21     
- Partials    18333    18336       +3     

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