fix: align regexp_instr empty-pattern behavior with PostgreSQL

[toby-bridges]

Summary

• remove the regexp_instr empty-pattern special case that forced 0
• rely on the existing regex search path so non-empty strings return PostgreSQL-compatible 1-based positions for empty patterns
• add scalar regression coverage and a sqllogictest case for SELECT regexp_instr('abc', '')

Fixes #22257.

Testing

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