public static String toJson(Message message) {
    StringWriter json = new StringWriter();
    try (JsonWriter jsonWriter = JsonWriter.of(json)) {
      write(message, jsonWriter);
    }
    return json.toString();
  }
@Test
  public void do_not_write_null_wrapper_of_map() {
    TestNullableMap msg = TestNullableMap.newBuilder()
      .setLabel("world")
      .build();
    assertThat(toJson(msg)).isEqualTo("{\"label\":\"world\"}");
  }