fix: support zero-field struct group keys in aggregate emit

[fornwall]

Which issue does this PR close?

Closes #25281.

Rationale for this change

GROUP BY and SELECT DISTINCT over a group key whose type contains a zero-field struct (Struct()) panicked when the aggregate emitted:

select arrow_cast(null, 'Struct()') as s, count(*) from (values (1),(2),(3)) group by 1;
-- thread 'tokio-rt-worker' panicked ... dictionary re-encode during emit:
-- InvalidArgumentError("use StructArray::try_new_with_length or StructArray::new_empty_fields ...")

The same panic hit SELECT DISTINCT and keys where the empty struct is nested inside another struct. A zero-field struct is a valid Arrow type and should group like any other key.

What changes are included in this PR?

encode_array_if_necessary in group_values/row.rs rebuilt every struct column with StructArray::try_new, which arrow rejects for zero fields since there is no child column to derive the length from. It now uses StructArray::try_new_with_length with the source array's length, so zero-field structs are rebuilt like any other struct. This function is shared by GroupValuesRows and the row-backed GroupColumn, so both paths are covered.

What is the testing strategy for this PR?

Three new sqllogictest cases at the end of struct.slt (whole-key, DISTINCT, and nested inside named_struct). All three panicked before the fix and pass after it.

Are there any user-facing changes?

No, other than the panic being fixed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.90%. Comparing base (15f32dd) to head (f2dd806).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25282      +/-   ##
==========================================
- Coverage   81.90%   81.90%   -0.01%     
==========================================
  Files        1134     1134              
  Lines      425200   425201       +1     
  Branches   425200   425201       +1     
==========================================
- Hits       348268   348251      -17     
- Misses      56291    56304      +13     
- Partials    20641    20646       +5     

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