[BUG] concat empty struct causes seg fault

[revans2]

Describe the bug
If I try to concatenate two structs columns with no children columns I get a segfault.

Steps/Code to reproduce bug
Apply the patch and run.

diff --git a/cpp/tests/copying/concatenate_tests.cu b/cpp/tests/copying/concatenate_tests.cu
index 7d3b7beb2c..e1c3ce1622 100644
--- a/cpp/tests/copying/concatenate_tests.cu
+++ b/cpp/tests/copying/concatenate_tests.cu
@@ -826,6 +826,19 @@ 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(5, {}, 0, rmm::device_buffer());
+
+  // concatenate
+  auto result = cudf::concatenate(std::vector<column_view>({*first, *second}));
+  cudf::test::expect_columns_equivalent(*result, *expected);
+}
+
 TEST_F(StructsColumnTest, ConcatenateSplitStructs)
 {
   using namespace cudf::test;

Expected behavior
It does not crash and ideally produces the correct answer.