fix: Properly write nested NullArray in Parquet

[coastalwhite]

Fixes #17795.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 10 lines in your changes missing coverage. Please review.

Project coverage is 80.49%. Comparing base (66f0026) to head (cbde319).
Report is 18 commits behind head on main.

Files	Patch %	Lines
crates/polars-arrow/src/bitmap/immutable.rs	86.53%	7 Missing ⚠️
crates/polars-utils/src/foreign_vec.rs	91.89%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17807      +/-   ##
==========================================
+ Coverage   80.47%   80.49%   +0.01%     
==========================================
  Files        1503     1504       +1     
  Lines      197115   197152      +37     
  Branches     2794     2805      +11     
==========================================
+ Hits       158628   158696      +68     
+ Misses      37973    37935      -38     
- Partials      514      521       +7     

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[coastalwhite]

There is something going wrong here in the propagate_nulls resulting from the struct refactor. @ritchie46 could you have a look at this?