public static <T> NullableCoder<T> of(Coder<T> valueCoder) {
    if (valueCoder instanceof NullableCoder) {
      return (NullableCoder<T>) valueCoder;
    }
    return new NullableCoder<>(valueCoder);
  }
@Test
  public void testCoderIsSerializableWithWellKnownCoderType() throws Exception {
    CoderProperties.coderSerializable(NullableCoder.of(GlobalWindow.Coder.INSTANCE));
  }