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.

[zeroshade]

Rebase onto #888 resolves the earlier build failure — binaryViewNoAllocAllocator and checkBinaryViewValueSize now exist. Verified on the PR head (7241331):

• go build ./arrow/array/ clean; TestBinaryViewBuilderReserveData0DoesNotAllocate, the oversized-value guard, and TestBinaryViewStringRoundTrip all pass.
• ./arrow/internal/arrjson and the IPC variadic-count test pass.

The ReserveData(0) short-circuit is safe: inline values never touch the block builder, and any non-inline value reserves with a positive length before UnsafeAppend. Dropping the unused empty data block also correctly removes the "VARIADIC_DATA_BUFFERS": [""] entries — Array.Variadic is omitempty, the writer derives the count from len(Buffers())-2, the reader loads a dynamic count, and BinaryView.setData only requires >=2 buffers with dataBuffers = buffers[2:]. This also aligns Go with the C++ behavior of not emitting an empty variadic buffer for all-inline data.

LGTM.