Fix logic in to_arrow for empty list column

[wence-]

Description

An empty list column need not have empty children, it just needs to have zero length. In this case, the offsets array will have zero length, and we need to create a temporary buffer.

Now that this branch runs, fix two errors in the construction of the arrow array:

1. The element type, if there are children, should be taken from the child array;
2. If the child arrays are empty, we must make an empty null array, rather than passing a null pointer as the values array, otherwise we hit a segfault inside arrow.

The previous fix in #16201 correctly handled the empty children case (except for point two), but not the first case, which we do here.

Since we we're previously going down this code path (child_arrays was never empty), we never hit the latent segfault from point two.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mythrocks]

LGTM.

[wence-]

/merge

[mroeschke]

Shouldn't the type of the empty child array be preserved here i.e. arrow::MakeEmptyArray or does that not matter?

[wence-]

aiui if the child_arrays are empty, then the list column had no children, so there's no information from libcudf on what the element type is. But perhaps @davidwendt to confirm.

[davidwendt]

Technically an empty list column would normally have a single empty child

cudf/cpp/src/lists/lists_column_factories.cu

Lines 86 to 94 in 04330f2

	std::unique_ptr<column> make_empty_lists_column(data_type child_type,
	rmm::cuda_stream_view stream,
	rmm::device_async_resource_ref mr)
	{
	auto offsets = make_empty_column(data_type(type_to_id<size_type>()));
	auto child = make_empty_column(child_type);
	return make_lists_column(
	0, std::move(offsets), std::move(child), 0, rmm::device_buffer{}, stream, mr);
	}

But it may also not have any child too.
I don't know if Arrow specifically cares but would bias towards what the callers of this API need.

[wence-]

Shouldn't the type of the empty child array be preserved here i.e. arrow::MakeEmptyArray or does that not matter?

To follow up, we are preserving the child array if child_arrays is not empty() (we use child_arrays[1] which will have the appropriate type. If child_arrays is empty() then there is no information available.

[wence-]

But, I went for best of both worlds by using MakeEmptyArray rather than hand-constructing the offset buffer.