Fix concatenate empty structs (rapidsai#8947)

Closes rapidsai#8929

Current PR is to fix the illegal memory access problem occurred when concatenating structs with empty children.

Authors:
  - Alfred Xu (https://github.com/sperlingxx)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Mike Wilson (https://github.com/hyperbolic2346)

URL: rapidsai#8947

cpp/src/structs/copying/concatenate.cu

@@ -28,6 +28,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <numeric>
 
 namespace cudf {
 namespace structs {
@@ -53,7 +54,11 @@ std::unique_ptr<column> concatenate(host_span<column_view const> columns,
                    return cudf::detail::concatenate(cols, stream, mr);
                  });
 
-  size_type const total_length = children[0]->size();
+  // get total length from concatenated children; if no child exists, we would compute it
+  auto const acc_size_fn = [](size_type s, column_view const& c) { return s + c.size(); };
+  auto const total_length =
+    !children.empty() ? children[0]->size()
+                      : std::accumulate(columns.begin(), columns.end(), size_type{0}, acc_size_fn);
 
   // if any of the input columns have nulls, construct the output mask
   bool const has_nulls =

cpp/tests/copying/concatenate_tests.cu

@@ -826,6 +826,22 @@ TEST_F(StructsColumnTest, ConcatenateStructs)
   cudf::test::expect_columns_equivalent(*result, *expected);
 }
 
+TEST_F(StructsColumnTest, ConcatenateEmptyStructs)
+{
+  using namespace cudf::test;
+
+  auto expected = cudf::make_structs_column(10, {}, 0, rmm::device_buffer());
+  auto first    = cudf::make_structs_column(5, {}, 0, rmm::device_buffer());
+  auto second   = cudf::make_structs_column(2, {}, 0, rmm::device_buffer());
+  auto third    = cudf::make_structs_column(0, {}, 0, rmm::device_buffer());
+  auto fourth   = cudf::make_structs_column(3, {}, 0, rmm::device_buffer());
+
+  // concatenate
+  auto result = cudf::concatenate(std::vector<column_view>({*first, *second, *third, *fourth}));
+  CUDF_EXPECTS(result->size() == expected->size(), "column size changed after concat");
+  cudf::test::expect_columns_equivalent(*result, *expected);
+}
+
 TEST_F(StructsColumnTest, ConcatenateSplitStructs)
 {
   using namespace cudf::test;