fix: preserve null bitmap in nested transform expressions

[DrakeLin]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/null-propagation [Files changed]
  • stack/coalesce [Files changed]
    • stack/checkpoint-transforms [Files changed]
      • stack/write-stats [Files changed]

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What changes are proposed in this pull request?

Fixes a bug in nested transform expression evaluation where null rows in the source struct were losing their null bitmap, causing null structs to incorrectly appear as non-null structs with null fields.

When evaluating nested transform expressions (transforms with an input_path that operate on a nested struct), the output StructArray was created with None for the null buffer:
let data = StructArray::try_new(output_fields.into(), output_cols, None)?;
This meant that if the source struct had null rows (e.g., an add action that is null in a checkpoint batch), the output would lose that null information. The struct would appear as non-null but with all-null fields, which is semantically different.

How was this change tested?

Existing transform tests pass. The stats transform integration tests (in a follow-up PR) exercise this code path.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.65%. Comparing base (daa9a1e) to head (37e7550).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/engine/arrow_expression/evaluate_expression.rs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1645   +/-   ##
=======================================
  Coverage   84.65%   84.65%           
=======================================
  Files         123      123           
  Lines       34109    34114    +5     
  Branches    34109    34114    +5     
=======================================
+ Hits        28875    28880    +5     
  Misses       3905     3905           
  Partials     1329     1329           

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[nicklan]

nice catch! i'm assuming from your PR comment that this is tested by code later in the stack, so lgtm

[dengsh12]

LGTM, thanks