Empty message structs should be allowed

[pchapin]

Right now it is necessary to include a "void field" in a message struct that is intended to be empty:

message struct -> Some_Request {
    void;
};

This is a bit odd looking since there is no field name, making it different than any other field. Furthermore, empty message structs are not particularly unusual. Often the existence of the message is sufficient, no additional message parameters are needed. Thus this odd construct appears somewhat frequently in MXDR interface definitions. It should be possible to define an empty message struct in a natural way:

message struct -> Some_Request {
};