No, you're right. EmptyMessage exists but you shouldn't make it an input or output to an RPC if that RPC might ever in the future accept or return arguments. Since people are super bad about predicting the future, just call it WhateverRequest and WhateverReply and if there are no arguments today, don't put any fields in those types.

I would also not use Any as a request or reply type, but Any, in general, is fairly common in practical use.