fix: Categorical min/max panicking when string cache is enabled

[lukemanley]

Fixes the following panic:

import polars as pl

pl.enable_string_cache()

pl.Series(["a", "b"], dtype=pl.Categorical()).min()
# pyo3_runtime.PanicException: internal error: entered unreachable code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.82%. Comparing base (b93b97b) to head (c85e3dc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #21552   +/-   ##
=======================================
  Coverage   79.82%   79.82%           
=======================================
  Files        1591     1591           
  Lines      229477   229477           
  Branches     2625     2625           
=======================================
+ Hits       183170   183178    +8     
+ Misses      45702    45694    -8     
  Partials      605      605           

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[ritchie46]

Thanks!