GH-48802: [C++] Replace redundant nullptr assignment to DCHECK_EQ in FixedSizeBinary to String/Binary cast

[HyukjinKwon]

Rationale for this change

The TODO comment saying explicitly setting output->buffers[2] = nullptr in the else branch was redundant.

The output ArrayData is allocated by PrepareOutput() before the kernel executes:

arrow/cpp/src/arrow/compute/exec.cc

Lines 907 to 908 in 7be5a89

	while (span_iterator_.Next(&input)) {
	ARROW_ASSIGN_OR_RAISE(output.value, PrepareOutput(input.length));

PrepareOutput() resizes the buffers vector to the required size for the output type (3 for String/Binary: validity, offsets, data):

arrow/cpp/src/arrow/compute/exec.cc

Lines 729 to 731 in 7be5a89

	Result<std::shared_ptr<ArrayData>> PrepareOutput(int64_t length) {
	auto out = std::make_shared<ArrayData>(output_type_.GetSharedPtr(), length);
	out->buffers.resize(output_num_buffers_);

For String/Binary output types, ComputeDataPreallocate() only preallocates the offsets buffer (not the data buffer):

arrow/cpp/src/arrow/compute/exec.cc

Lines 294 to 298 in 7be5a89

	case Type::BINARY:
	case Type::STRING:
	case Type::LIST:
	case Type::MAP:
	widths->emplace_back(32, /*added_length=*/1);

PrepareOutput() only allocates buffers that are in the data_preallocated_ vector. For String/Binary, this means only buffers[1] (offsets) is allocated:

arrow/cpp/src/arrow/compute/exec.cc

Lines 739 to 747 in 7be5a89

	for (size_t i = 0; i < data_preallocated_.size(); ++i) {
	const auto& prealloc = data_preallocated_[i];
	if (prealloc.bit_width >= 0) {
	ARROW_ASSIGN_OR_RAISE(
	out->buffers[i + 1],
	AllocateDataBuffer(kernel_ctx_, length + prealloc.added_length,
	prealloc.bit_width));
	}
	}

Since std::vector::resize()constructs all elements, buffers[2] is already nullptr from the resize call. Therefore, explicitly assigning nullptr in the else branch is redundant.

What changes are included in this PR?

Converted the redundant else clause to DCHECK_EQ.

Are these changes tested?

Yes, I locally ranCast.FixedSizeBinaryToBinaryOrString and Cast.FixedSizeBinaryToBinaryOrStringWithSlice.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Remove redundant nullptr assignment in FixedSizeBinary to String/Binary cast #48802

[github-actions]

⚠️ GitHub issue #48802 has been automatically assigned in GitHub to PR creator.