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.

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