A webservice should always callback to the original application. Otherwise the application can't see the difference between the lack of a connection or a succesfull call.
So the webservice should at least send a soap envelope (the tech name of the message). But this envelope can be empty.
In the modeler you can publish a microflow which returns nothing. This will return an empty response. But it still returns a response to let the other application know the call was succesfull.