fix: Materialize dtypes when converting to arrow

[ritchie46]

This should fix #16021

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 80.98%. Comparing base (421db23) to head (db3e318).

Files	Patch %	Lines
crates/polars-core/src/datatypes/dtype.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16074   +/-   ##
=======================================
  Coverage   80.97%   80.98%           
=======================================
  Files        1386     1386           
  Lines      178445   178452    +7     
  Branches     3075     3075           
=======================================
+ Hits       144503   144518   +15     
+ Misses      33450    33443    -7     
+ Partials      492      491    -1     

Flag	Coverage Δ
python	74.54% <0.00%> (+0.02%)	⬆️
rust	78.14% <0.00%> (+<0.01%)	⬆️

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