daemon/worker send_waiting(): be more defensive

daemon/worker send_waiting(): be more defensive

We encountered non-recoverable assertions due to popping
from an empty queue here, but I see no reason to block recovery here.
I'm still keeping it as a soft assertion until it's better understood.

I *suspect* what happened is that:
 - multiple queries queued up before outgoing TCP handshake completed
 - the session got into closing state for some reason
   *before* processing this whole queue
 - during that the queue got emptied