perf: Add dedicated no-null branch in arg_sort

[ritchie46]

Slightly less work by adding a non-null branch. Will follow up with a proper speed up and special implementation for view types and binaryoffset (row encoding).

[codspeed-hq]

CodSpeed Performance Report

Merging #16808 will not alter performance

_{Comparing sort (2b185c6) with main (a0a577a)}

Summary

✅ 37 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 93.06931% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.39%. Comparing base (a0a577a) to head (2b185c6).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...ates/polars-core/src/chunked_array/ops/sort/mod.rs	88.52%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16808      +/-   ##
==========================================
+ Coverage   81.37%   81.39%   +0.01%     
==========================================
  Files        1424     1424              
  Lines      187554   187579      +25     
  Branches     2697     2697              
==========================================
+ Hits       152622   152672      +50     
+ Misses      34435    34410      -25     
  Partials      497      497              

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