fix: align regexp_instr empty pattern

[Sean-Kenneth-Doherty]

Which issue does this PR close?

• Closes PostgreSQL compatibility: regexp_instr with an empty pattern should return 1 #22257.

Rationale for this change

PostgreSQL treats an empty regular expression in regexp_instr as a zero-width match. DataFusion previously special-cased empty patterns to return 0, so regexp_instr('abc', '') diverged from PostgreSQL.

What changes are included in this PR?

• Handles empty regexp_instr patterns as zero-width matches at start + N - 1, returning 0 when that position is past the end of the string.
• Keeps existing regex/flag validation for empty patterns.
• Lets start/N validation run before empty-string handling.
• Adds Rust unit coverage and sqllogictest coverage for empty-pattern behavior.

Are these changes tested?

• cargo fmt --all
• cargo test -p datafusion-functions regex::regexpinstr::tests::test_regexp_instr -- --nocapture
• cargo test --profile=ci --test sqllogictests -- regexp/regexp_instr.slt
• TMPDIR=/home/sean/Projects/datafusion-contrib/target/tmp cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Are there any user-facing changes?

Yes. regexp_instr now matches PostgreSQL behavior for empty regular expression patterns.