fix(rust): Handle AnyValue::Struct to prevent null returns

[i64]

Fixes #18800.

[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.85%. Comparing base (25f84e4) to head (54b6640).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/series/any_value.rs	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18801      +/-   ##
==========================================
+ Coverage   79.84%   79.85%   +0.01%     
==========================================
  Files        1518     1518              
  Lines      205577   205595      +18     
  Branches     2892     2892              
==========================================
+ Hits       164144   164184      +40     
+ Misses      40885    40863      -22     
  Partials      548      548              

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[ritchie46]

Thanks!