fix: Cast the column and fill value to their common supertype

[I-Al-Istannen]

This ensures columns without any data (represented as the Null type) get the dtype of their replacement value in fill_null. This PR also adds a minimal testcase for the problem.

Closes #19738

[codecov]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.81%. Comparing base (823bfc9) to head (ec912b3).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...tes/polars-plan/src/dsl/function_expr/fill_null.rs	46.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21509      +/-   ##
==========================================
+ Coverage   79.73%   79.81%   +0.08%     
==========================================
  Files        1605     1592      -13     
  Lines      230053   229280     -773     
  Branches     2623     2623              
==========================================
- Hits       183424   182996     -428     
+ Misses      46024    45679     -345     
  Partials      605      605              

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[ritchie46]

This isn't the proper solution. We need dynamic nested types instead of band aiding this in the fill_null.