fix: Fix struct arithmetic schema

[ritchie46]

fixes #16373

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.41%. Comparing base (d1cf113) to head (9219e44).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...rates/polars-plan/src/logical_plan/aexpr/schema.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16396      +/-   ##
==========================================
+ Coverage   81.40%   81.41%   +0.01%     
==========================================
  Files        1409     1409              
  Lines      184509   184513       +4     
  Branches     2961     2961              
==========================================
+ Hits       150199   150221      +22     
+ Misses      33795    33777      -18     
  Partials      515      515              

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