code: public void setMessageId(String messageId) {
          if (messageId == null) {
              _properties.remove(AMQP_PROP_MESSAGE_ID);
              return;
          }
          
          _properties.put(AMQP_PROP_MESSAGE_ID, messageId);
      }
docs: Sets the value of "messageId" property. If a null value is passed
  in, it indicates that the property is not set.
  
  @param  messageId    value of "messageId" property