fix(common): support empty struct in ScalarValue::compact and new_default

[unikdahal]

Which issue does this PR close?

Closes#24581.

Rationale for this change

StructArray::new panics for structs with zero fields because it cannot infer array length from an empty column list. This hit two code paths in ScalarValue: new_default when building a default value for a zero-field struct type, and compact_view_buffers (used by ScalarValue::compact) when compacting a struct array that has zero fields but a nonzero row count.

What changes are included in this PR?

• ScalarValue::new_default and compact_view_buffers now special-case zero-field structs, using StructArray::new_empty_fields (which takes an explicit length/nulls instead of inferring it from columns) rather than StructArray::new.
• Added test_compact_empty_struct covering the panic case.

Are these changes tested?

Yes, new unit test added.

Are there any user-facing changes?

No behavior change other than fixing a panic on empty-field structs.

[Jefffrey]

CI looks to be failing

[Jefffrey]

could probably clone the input instead of reconstructing?

[Jefffrey]

seems a bit odd that a scalar would have 3 rows

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.59259% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.42%. Comparing base (5134a1a) to head (6cd684f).
⚠️ Report is 59 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/common/src/scalar/mod.rs	92.59%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #24582 +/- ##
==========================================
+ Coverage 81.38% 81.42% +0.04% 
==========================================
Files 1116 1121 +5 Lines 397960 402202 +4242 Branches 397960 402202 +4242 ==========================================
+ Hits 323880 327502 +3622 - Misses 55120 55498 +378 - Partials 18960 19202 +242 

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[unikdahal]

Thanks @Jefffrey!

Empty struct handling: Since zero-field structs don't have child buffers to compact, compact_view_buffers now just returns the input array directly.

Test update: Switched test_compact_empty_struct from the 3-row scalar to a single-row StructArray (kept a null row so we still verify null buffer handling).

CI: Fixed the cargo fmt issue in the new nested test.

[Jefffrey]

thanks @unikdahal