[CHORE]: prepare for nulls first/last kernels

[universalmind303]

This pr adds nulls_first to all sort functions in preparation for implementing the nulls first/last kernels. This will be followed up by adding the actual nulls first/last implementations.

[codspeed-hq]

CodSpeed Performance Report

Merging #3301 will degrade performances by 35.59%

_{Comparing universalmind303:nulls-first (f6593b9) with main (e18b719)}

Summary

❌ 1 regressions
✅ 16 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	universalmind303:nulls-first	Change
❌	test_show[100 Small Files]	14.9 ms	23.2 ms	-35.59%