fix: Allow .get(null) in groupby context

[orlp]

Fixes #15163.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.19%. Comparing base (27289b2) to head (6b51d25).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/expressions/gather.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #19401    +/-   ##
========================================
  Coverage   80.18%   80.19%            
========================================
  Files        1523     1527     +4     
  Lines      209897   210286   +389     
  Branches     2434     2440     +6     
========================================
+ Hits       168314   168630   +316     
- Misses      41028    41101    +73     
  Partials      555      555            

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