fix: properly support nulls_last + descending

[orlp]

Fixes #15205.

[codecov]

Codecov Report

Attention: Patch coverage is 92.68293% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 81.25%. Comparing base (0bd6ac8) to head (fafacf4).
Report is 9 commits behind head on main.

❗ Current head fafacf4 differs from pull request most recent head edefe0a. Consider uploading reports for the commit edefe0a to get more accurate results

Files	Patch %	Lines
...polars-core/src/chunked_array/ops/compare_inner.rs	92.30%	2 Missing ⚠️
...olars-lazy/src/physical_plan/expressions/sortby.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15212      +/-   ##
==========================================
- Coverage   81.25%   81.25%   -0.01%     
==========================================
  Files        1354     1354              
  Lines      175427   175664     +237     
  Branches     2518     2518              
==========================================
+ Hits       142545   142737     +192     
- Misses      32401    32446      +45     
  Partials      481      481              

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