Fix writing of ORC files with empty child string columns (#13745)

Closes #13742
Fixes an OOB access in `rowgroup_char_counts_kernel` when the input column has no rows. This can happen with string columns with a parent list column.

Authors:
  - Vukasin Milovanovic (https://github.com/vuule)

Approvers:
  - Robert (Bobby) Evans (https://github.com/revans2)
  - Karthikeyan (https://github.com/karthikeyann)
  - Mike Wilson (https://github.com/hyperbolic2346)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #13745

cpp/src/io/orc/dict_enc.cu

@@ -45,7 +45,9 @@ __global__ void rowgroup_char_counts_kernel(device_2dspan<size_type> char_counts
 
   auto const& offsets = str_col.child(strings_column_view::offsets_column_index);
   char_counts[str_col_idx][row_group_idx] =
-    offsets.element<size_type>(start_row + num_rows) - offsets.element<size_type>(start_row);
+    (num_rows == 0)
+      ? 0
+      : offsets.element<size_type>(start_row + num_rows) - offsets.element<size_type>(start_row);
 }
 
 void rowgroup_char_counts(device_2dspan<size_type> counts,

cpp/tests/io/orc_test.cpp

@@ -1844,4 +1844,21 @@ TEST_F(OrcWriterTest, SlicedStringColumn)
   CUDF_TEST_EXPECT_TABLES_EQUAL(expected_slice, result.tbl->view());
 }
 
+TEST_F(OrcWriterTest, EmptyChildStringColumn)
+{
+  list_col<cudf::string_view> col{{}, {}};
+  table_view expected({col});
+
+  auto filepath = temp_env->get_temp_filepath("OrcEmptyChildStringColumn.orc");
+  cudf::io::orc_writer_options out_opts =
+    cudf::io::orc_writer_options::builder(cudf::io::sink_info{filepath}, expected);
+  cudf::io::write_orc(out_opts);
+
+  cudf::io::orc_reader_options in_opts =
+    cudf::io::orc_reader_options::builder(cudf::io::source_info{filepath}).use_index(false);
+  auto result = cudf::io::read_orc(in_opts);
+
+  CUDF_TEST_EXPECT_TABLES_EQUAL(expected, result.tbl->view());
+}
+
 CUDF_TEST_PROGRAM_MAIN()