There is no out-of-the-box way to achieve this kind of functionality in Prism. The simple workaround is firing an event with a payload that means nothing (make it an object an always send null).
If you definitely require that the event is sent without a payload, you could have a class inherit from EventBase and make a copy most code from CompositePresentationEvent<T>, but make it non-generic. Take into account that it is not the only class
you will have to modify, as some other classes depend on the generic payload.
Please let me know if this helps.