[BUG] struct row comparator crashes on empty structs

[sperlingxx]

Describe the bug
The latest struct row comparator, which was introduced by #10164, leads to CUDA_ERROR when encountering empty structs.

Steps/Code to reproduce bug

  std::vector<bool> v{0, 0, 0, 0, 0, 0};
  fixed_width_column_wrapper<int32_t> col_for_mask{{0, 0, 0, 0, 0, 0}, v.begin()};
  auto null_mask  = cudf::copy_bitmask(col_for_mask.release()->view());
  auto struct_col = cudf::make_structs_column(
    6, std::vector<std::unique_ptr<column>>{}, UNKNOWN_NULL_COUNT, std::move(null_mask));

  fixed_width_column_wrapper<int32_t> col1{{1, 2, 3, 1, 2, 3}};
  fixed_width_column_wrapper<int32_t> col2{{1, 1, 1, 2, 2, 2}};
  table_view input{{col1, col2, struct_col->view()}};

  fixed_width_column_wrapper<int32_t> expected{{0, 2, 4, 1, 3, 5}};
  std::vector<order> column_order{order::ASCENDING, order::ASCENDING, order::ASCENDING};

  auto got = sorted_order(input, column_order);
  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, got->view());