fix: count empty regexp matches in regexp_count

[toby-bridges]

Summary

• stop short-circuiting regexp_count when the pattern is an empty string
• preserve NULL regex handling for scalar inputs while allowing empty patterns to flow through the normal regex counting path
• add a scalar regression test and a sqllogictest case for SELECT regexp_count('abc', '')

Fixes #22267.

Testing

• cargo test -p datafusion-functions regexp_count
• cargo test -p datafusion-sqllogictest --test sqllogictests -- regexp_count.slt
• cargo fmt --all --check
• git diff --check

[kosiew]

@toby-bridges,
Thanks for your contribution.

Closing this as it is #22311 already addesses the issue.