swarm-derive/: Do not default out_event to () when event_process=false

[mxinden]

By default the NetworkBehaviour derive macro out_event attribute is (). This makes sense when the event_process attribute is set to true, though with #2214 the NetworkBehaviour derive macro event_process attribute is false by default. Not processing the event (event_process=false) and setting an empty out event (()) defeats the purpose of the event mechanism of the NetworkBehaviour trait.

I suggest requiring a user to set out_event to a custom enum when event_process is false.

This has been raised by @thomaseizinger in #2361 (comment).

[thomaseizinger]

We now automatically generate the out event.