fix: Fix invalid alias udf

[ritchie46]

fixes #18914

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.12%. Comparing base (ab5200d) to head (65dbd74).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-python/src/map/series.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19021       +/-   ##
===========================================
- Coverage   79.85%   60.12%   -19.74%     
===========================================
  Files        1524     1524               
  Lines      207734   207747       +13     
  Branches     2908     2908               
===========================================
- Hits       165891   124911    -40980     
- Misses      41294    82287    +40993     
  Partials      549      549               

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