fix: Don't panic but set null type if type is unknown

[ritchie46]

Needed for cloud.

[github-actions]

The uncompressed lib size after this PR is 1115.6178 MB.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.02%. Comparing base (247f0b1) to head (26d7375).
Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/series/ops/null.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20647      +/-   ##
==========================================
- Coverage   79.03%   79.02%   -0.02%     
==========================================
  Files        1557     1557              
  Lines      220528   220552      +24     
  Branches     2510     2514       +4     
==========================================
- Hits       174303   174281      -22     
- Misses      45651    45698      +47     
+ Partials      574      573       -1     

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