fix(arrow/scalar): handle zero-length scalar arrays

[fallintoplace]

Rationale for this change

MakeArrayFromScalar accepts a length of zero, but nested scalar types try to concatenate an empty input. Run-end encoded scalars also need an empty physical representation.

What changes are included in this PR?

Use empty builders for zero-length output and construct run-end encoded arrays with zero runs. Add coverage for nested and run-end encoded scalars.

Are these changes tested?

• go test ./arrow/scalar

Are there any user-facing changes?

No API changes. This corrects the reported behavior while preserving the existing ownership and compatibility contracts.

[zeroshade]

The zero-length handling is correct. Empty arrays retain their data types, validate fully, and clean up correctly across nested, dictionary, union, extension, and run-end encoded scalars. The focused, race, 32-bit, vet, and CI checks pass.

Please rebase this branch onto current main before merging. Both touched files now conflict; the resolution should retain this zero-length branch and tests alongside main’s newer MakeArrayFromScalar offset-width fixes and associated tests.

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