What if publishing message doesn’t require any data to be passed?

When working with message queues we define a topic. The request or a schema argument is required. But what if I don’t need any parameters to be passed in? I can’t really pass a null or a void in there; it must be something. But I don’t really want to pass anything dummy. What are my alternatives?

For instance, let’s say one wants to flush the cache programatically. The consumer can execute this code without any arguments needed to be passed.

Thanks in advance