R1:e2f057110462

Drop empty inbound mail at the beginning of the receive workflow, not inside object handlers

Summary:
Ref T920. Ref T7477. We currently drop empty mail only once it reaches the `ReplyHandler` layer.

I think no plausible receiver can ever do anything useful with this kind of mail, so we can safely drop it earlier and simplify some of the logic. After T7477, we'd end up throwing multiple exceptions if you sent empty mail to several valid receivers.

(I also want to move away from APIs oriented around raw…