fix: Incorrect result for merge_sorted with lexical categorical

[lukemanley]

fixes #21025

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.89%. Comparing base (4eb4fb7) to head (b6b0227).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21278      +/-   ##
==========================================
- Coverage   79.89%   79.89%   -0.01%     
==========================================
  Files        1596     1596              
  Lines      228552   228560       +8     
  Branches     2608     2608              
==========================================
+ Hits       182607   182611       +4     
- Misses      45349    45353       +4     
  Partials      596      596              

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