fix: Struct outer nullabillity

[ritchie46]

Fix more struct out nullabillity bugs.

fixes #18119

[codecov]

Codecov Report

Attention: Patch coverage is 96.05263% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (8c19d18) to head (ba7322d).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/polars-core/src/chunked_array/ops/zip.rs	95.23%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18156      +/-   ##
==========================================
+ Coverage   80.34%   80.37%   +0.02%     
==========================================
  Files        1496     1496              
  Lines      197809   197871      +62     
  Branches     2822     2822              
==========================================
+ Hits       158934   159031      +97     
+ Misses      38353    38318      -35     
  Partials      522      522              

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