fix: Fix empty output of to_arrow() on filtered unit height DataFrame

[nameexhaustion]

Fixes #20627

Fix Column::n_chunks()/rechunk() to account for the chunks in the cached materialized Series for the scalar case, which can be created if ScalarColumn::from_single_value_series() is called with a Series with >1 chunks.

[codecov]

Codecov Report

Attention: Patch coverage is 52.38095% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (8abd261) to head (5bde175).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/column/mod.rs	52.38%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20656   +/-   ##
=======================================
  Coverage   78.97%   78.97%           
=======================================
  Files        1557     1557           
  Lines      220696   220714   +18     
  Branches     2517     2517           
=======================================
+ Hits       174297   174312   +15     
- Misses      45825    45828    +3     
  Partials      574      574           

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