perf: Optimize split_part using bulk-NULL string builders

[neilconway]

Which issue does this PR close?

• Closes Optimize split_part using bulk-NULL string builders #22282.

Rationale for this change

split_part currently uses the Arrow StringBuilder types and computes NULLs on a per-row basis. This PR switches to using the new bulk-NULL string builders.

Benchmarks (Arm64):

• scalar_utf8_single_char / pos_first: 44.6 µs → 39.1 µs (−11.2%)
• scalar_utf8_single_char / pos_middle: 102.6 µs → 95.8 µs (−6.4%)
• scalar_utf8_single_char / pos_negative: 48.6 µs → 42.5 µs (−12.4%)
• scalar_utf8_multi_char / pos_middle: 134.1 µs → 130.4 µs (−2.9%)
• scalar_utf8_long_strings / pos_middle: 1089 µs → 1101 µs (+1.3%, within noise)
• scalar_utf8view_long_parts / pos_middle: 140.6 µs → 138.0 µs (−2.0%, within noise)
• scalar_utf8view_very_long_parts / pos_first: 68.9 µs → 69.4 µs (+1.3%, within noise)
• array_utf8_single_char / pos_middle: 360.2 µs → 346.6 µs (−3.9%)
• array_utf8_multi_char / pos_middle: 354.3 µs → 343.2 µs (−2.2%, borderline)

What changes are included in this PR?

• Switch to new string builder types; compute NULLs in bulk via NullBuffer::union_many

Are these changes tested?

Yes, covered by existing tests.

Are there any user-facing changes?

No.