Fix logic in to_arrow for empty list column (#16279)

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.

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - MithunR (https://github.com/mythrocks)

URL: #16279

cpp/src/interop/to_arrow.cu

@@ -378,13 +378,11 @@ std::shared_ptr<arrow::Array> dispatch_to_arrow::operator()<cudf::list_view>(
   auto children_meta =
     metadata.children_meta.empty() ? std::vector<column_metadata>{{}, {}} : metadata.children_meta;
   auto child_arrays = fetch_child_array(input_view, children_meta, ar_mr, stream);
-  if (child_arrays.empty()) {
-    // Empty list will have only one value in offset of 4 bytes
-    auto tmp_offset_buffer = allocate_arrow_buffer(sizeof(int32_t), ar_mr);
-    memset(tmp_offset_buffer->mutable_data(), 0, sizeof(int32_t));
-
-    return std::make_shared<arrow::ListArray>(
-      arrow::list(arrow::null()), 0, std::move(tmp_offset_buffer), nullptr);
+  if (child_arrays.empty() || child_arrays[0]->data()->length == 0) {
+    auto element_type = child_arrays.empty() ? arrow::null() : child_arrays[1]->type();
+    auto result       = arrow::MakeEmptyArray(arrow::list(element_type), ar_mr);
+    CUDF_EXPECTS(result.ok(), "Failed to construct empty arrow list array\n");
+    return result.ValueUnsafe();
   }
 
   auto offset_buffer = child_arrays[0]->data()->buffers[1];