fix(python): Fix fill null types

[ritchie46]

fixes #19651

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.93%. Comparing base (0f64785) to head (474ffe5).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/lazyframe/frame.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19656      +/-   ##
==========================================
+ Coverage   79.92%   79.93%   +0.01%     
==========================================
  Files        1536     1536              
  Lines      211686   211718      +32     
  Branches     2445     2446       +1     
==========================================
+ Hits       169192   169241      +49     
+ Misses      41939    41923      -16     
+ Partials      555      554       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.