Sorry, I'm pretty sure there's already a plugin to do this, but I haven't been able to find it or figure out what to search for:
When somebody joins an empty server (that used to have someone on it), the round/map has run out of time, and there's an instant "YOU'VE FAILED: STALEMATE" or "PREPARE FOR SUDDEN DEATH".
Is there any way to prevent this? Ideally, a map should restart when the first player joins.