code: public ImmutableList<SoyMsgPart> getMsgParts(long msgId) {
      SoyMsg msg = getMsg(msgId);
      return msg == null ? ImmutableList.of() : msg.getParts();
    }
docs: Returns the message parts, or an empty array if there is no such message.
  
  <p>This is useful for rendering only usecases when the rest of the {@link SoyMsg} doesn't
  matter. The default implementation is just {@link SoyMsg#getParts} but some subclasses may have
  more efficient implementations