fix: Add outer validity to AnyValueBufferTrusted for structs

[orlp]

This technically changes behavior for df.pivot (possibly other things), but the old behavior was honestly bugged (despite us having a test for it).

Old behavior:

>>> df
┌─────┬──────┬──────┬──────┬───────────┐
│ id  ┆ week ┆ num1 ┆ num2 ┆ nums      │
│ --- ┆ ---  ┆ ---  ┆ ---  ┆ ---       │
│ str ┆ str  ┆ i64  ┆ i64  ┆ struct[2] │
╞═════╪══════╪══════╪══════╪═══════════╡
│ a   ┆ 1    ┆ 1    ┆ 4    ┆ {1,4}     │
│ a   ┆ 2    ┆ 3    ┆ 5    ┆ {3,5}     │
│ b   ┆ 3    ┆ 5    ┆ 3    ┆ {5,3}     │
│ c   ┆ 4    ┆ 4    ┆ 4    ┆ {4,4}     │
│ c   ┆ 3    ┆ 3    ┆ 6    ┆ {3,6}     │
│ c   ┆ 1    ┆ 6    ┆ 6    ┆ {6,6}     │
└─────┴──────┴──────┴──────┴───────────┘
>>> df.pivot(values="nums", index="id", on="week", aggregate_function="first")
┌─────┬─────────────┬─────────────┬─────────────┬─────────────┐
│ id  ┆ 1           ┆ 2           ┆ 3           ┆ 4           │
│ --- ┆ ---         ┆ ---         ┆ ---         ┆ ---         │
│ str ┆ struct[2]   ┆ struct[2]   ┆ struct[2]   ┆ struct[2]   │
╞═════╪═════════════╪═════════════╪═════════════╪═════════════╡
│ a   ┆ {1,4}       ┆ {3,5}       ┆ {null,null} ┆ {null,null} │
│ b   ┆ {null,null} ┆ {null,null} ┆ {5,3}       ┆ {null,null} │
│ c   ┆ {6,6}       ┆ {null,null} ┆ {3,6}       ┆ {4,4}       │
└─────┴─────────────┴─────────────┴─────────────┴─────────────┘

New behavior:

┌─────┬───────────┬───────────┬───────────┬───────────┐
│ id  ┆ 1         ┆ 2         ┆ 3         ┆ 4         │
│ --- ┆ ---       ┆ ---       ┆ ---       ┆ ---       │
│ str ┆ struct[2] ┆ struct[2] ┆ struct[2] ┆ struct[2] │
╞═════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ a   ┆ {1,4}     ┆ {3,5}     ┆ null      ┆ null      │
│ b   ┆ null      ┆ null      ┆ {5,3}     ┆ null      │
│ c   ┆ {6,6}     ┆ null      ┆ {3,6}     ┆ {4,4}     │
└─────┴───────────┴───────────┴───────────┴───────────┘

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.92%. Comparing base (1cd72ff) to head (b96effd).
Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/row/av_buffer.rs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20713   +/-   ##
=======================================
  Coverage   79.92%   79.92%           
=======================================
  Files        1559     1559           
  Lines      221181   221178    -3     
  Branches     2530     2530           
=======================================
+ Hits       176774   176784   +10     
+ Misses      43825    43812   -13     
  Partials      582      582           

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