Fix write_json to handle empty string column (#16995)

Add empty string column condition for write_json 
bypass make_strings_children for empty column because when grid size is zero, it throws cuda error.

Authors:
  - Karthikeyan (https://github.com/karthikeyann)
  - Muhammad Haseeb (https://github.com/mhaseeb123)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Muhammad Haseeb (https://github.com/mhaseeb123)

URL: #16995

cpp/src/io/json/write_json.cu

@@ -170,6 +170,9 @@ struct escape_strings_fn {
  rmm::cuda_stream_view stream,
  rmm::device_async_resource_ref mr)
  {
+ if (column_v.is_empty()) { // empty begets empty
+ return make_empty_column(type_id::STRING);
+ }
  auto [offsets_column, chars] =
  cudf::strings::detail::make_strings_children(*this, column_v.size(), stream, mr);
 

cpp/tests/io/json/json_writer.cpp

@@ -70,6 +70,43 @@ TEST_F(JsonWriterTest, EmptyInput)
  EXPECT_EQ(expected_lines, std::string(out_buffer.data(), out_buffer.size()));
 }
 
+TEST_F(JsonWriterTest, EmptyLeaf)
+{
+ cudf::test::strings_column_wrapper col1{""};
+ cudf::test::fixed_width_column_wrapper<cudf::size_type> offsets{0, 0};
+ auto col2 = make_lists_column(1,
+ offsets.release(),
+ cudf::test::strings_column_wrapper{}.release(),
+ 0,
+ rmm::device_buffer{},
+ cudf::test::get_default_stream());
+ auto col3 = cudf::test::lists_column_wrapper<int>::make_one_empty_row_column();
+ cudf::table_view tbl_view{{col1, *col2, col3}};
+ cudf::io::table_metadata mt{{{"col1"}, {"col2"}, {"col3"}}};
+
+ std::vector<char> out_buffer;
+ auto destination = cudf::io::sink_info(&out_buffer);
+ auto out_options = cudf::io::json_writer_options_builder(destination, tbl_view)
+ .include_nulls(true)
+ .metadata(mt)
+ .lines(false)
+ .na_rep("null")
+ .build();
+
+ // Empty columns in table
+ cudf::io::write_json(out_options, cudf::test::get_default_stream());
+ std::string const expected = R"([{"col1":"","col2":[],"col3":[]}])";
+ EXPECT_EQ(expected, std::string(out_buffer.data(), out_buffer.size()));
+
+ // Empty columns in table - JSON Lines
+ out_buffer.clear();
+ out_options.enable_lines(true);
+ cudf::io::write_json(out_options, cudf::test::get_default_stream());
+ std::string const expected_lines = R"({"col1":"","col2":[],"col3":[]})"
+ "\n";
+ EXPECT_EQ(expected_lines, std::string(out_buffer.data(), out_buffer.size()));
+}
+
 TEST_F(JsonWriterTest, ErrorCases)
 {
  cudf::test::strings_column_wrapper col1{"a", "b", "c"};