Reset title when an empty title string is given

Reset title when an empty title string is given

With this patch, st will reset its window title when an empty string is
given as the terminal title. For example:
	printf "\033]0;\007"

Some applications, like termdown, expect this functionality. xterm
implements it, but it seems that most other terminal emulators don't.
In any case, I don't see why there should ever be a case where the st
window doesn't have a title property.