fix: handle null lists with empty children in Spark's shuffle

[neilconway]

Which issue does this PR close?

• N/A

Rationale for this change

The Spark-compatible shuffle implementation unnecessarily copied the first child element as a placeholder for each null list. This is wrong for typed-NULL lists with no children, and was redundant work anyway: NULL lists can be adequately represented by a repeated offset entry, along with setting the appropriate bit in the validity buffer.

What changes are included in this PR?

• Fix bug
• Add unit test

What is the testing strategy for this PR?

Existing tests pass, new test added.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.39%. Comparing base (24a7f13) to head (1bbcae1).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/array/shuffle.rs	80.76%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             main   #25544     +/-   ##
=========================================
  Coverage   82.38%   82.39%             
=========================================
  Files        1138     1138             
  Lines      433739   434746   +1007     
  Branches   433739   434746   +1007     
=========================================
+ Hits       357338   358203    +865     
- Misses      54847    54855      +8     
- Partials    21554    21688    +134     

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