fix: Fix map_elements ignoring skip_nulls=True for struct dtype

[lukemanley]

closes #10102

import polars as pl

df = pl.DataFrame(
    {
        "a": [{"b": 1}, None],
    }
)

df.select(
    pl.col("a").map_elements(lambda x: x["b"], skip_nulls=True)
)

main:

polars.exceptions.ComputeError: TypeError: 'NoneType' object is not subscriptable

This PR:

shape: (2, 1)
┌──────┐
│ a    │
│ ---  │
│ i64  │
╞══════╡
│ 1    │
│ null │
└──────┘

[codecov]

Codecov Report

Attention: Patch coverage is 78.04878% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (87baf86) to head (52e7193).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-python/src/map/series.rs	74.28%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20668      +/-   ##
==========================================
+ Coverage   78.85%   78.99%   +0.13%     
==========================================
  Files        1558     1558              
  Lines      221055   221083      +28     
  Branches     2527     2527              
==========================================
+ Hits       174318   174647     +329     
+ Misses      46159    45858     -301     
  Partials      578      578              

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[ritchie46]

Nice, can we add a test where we test both ignore as non-ignore case?