fix: Check groups in group-by filter

[ritchie46]

fixes #18295

[codecov]

Codecov Report

Attention: Patch coverage is 88.09524% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.19%. Comparing base (0f0e12b) to head (d2c9fc7).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-expr/src/expressions/mod.rs	0.00%	3 Missing ⚠️
crates/polars-expr/src/expressions/gather.rs	90.00%	1 Missing ⚠️
crates/polars-expr/src/expressions/ternary.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18300   +/-   ##
=======================================
  Coverage   80.19%   80.19%           
=======================================
  Files        1501     1501           
  Lines      198944   198943    -1     
  Branches     2837     2837           
=======================================
+ Hits       159543   159551    +8     
+ Misses      38873    38864    -9     
  Partials      528      528           

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