fix(arrow/array): avoid allocating for ReserveData(0)

[fallintoplace]

Rationale for this change

BinaryViewBuilder.ReserveData always delegated to the block builder. ReserveData(0) could therefore create a full default data block even when every value remains inline.

What changes are included in this PR?

• Return after the existing size validation when the requested data length is zero.
• Preserve the normal reserve path for positive lengths.
• Update integration JSON expectations because an unused empty variadic data buffer is no longer created.

Are these changes tested?

Yes. A no-allocation allocator verifies that ReserveData(0) does not allocate. The array and arrjson packages pass normal and race tests.

Are there any user-facing changes?

Zero-length reservations no longer allocate an unused data block. Positive reservations and the public API are unchanged.

[zeroshade]

The ReserveData(0) short-circuit is reasonable, but the branch doesn't compile on its own:

arrow/array/binary_test.go:705:28: undefined: binaryViewNoAllocAllocator
FAIL	github.com/apache/arrow-go/v18/arrow/array [build failed]

binaryViewNoAllocAllocator is introduced in #888 — it's not on main and not in this PR. Please rebase this on top of #888 (so the helper exists) or add the helper here. Once it builds, the guard + test look good.

Verified by checking out the PR head and running go test ./arrow/array/.

[zeroshade]

Rebased onto main now that #888 has merged (it introduced checkBinaryViewValueSize and the binaryViewNoAllocAllocator test helper). Resolved the ReserveData and binary_test.go conflicts, and confirmed go build/go test ./arrow/array/ are green (both TestBinaryViewBuilderReserveData0DoesNotAllocate and the oversized-value guard pass). This should now compile — over to CI.