creating a popup w/ an empty string leaves blank lines behind

[BinaryKhaos]

While I was debugging jorgenschaefer/elpy#182, I dived deep into auto-complete.el and popup.el to find the cause for the "permanently" added empty lines I was seeing w/ elpy.

In the case of elpy, auto-complete queried for a documentation string and got an empty string which it used to create a quick tip (ac-quick-help -> popup-menu-show-quick-help -> popup-tip -> popup-create).

Most likely, a popup w/ width 0 is created, which will cause trouble in some conditional blocks. I haven't had the time to debug this even further, I am afraid. But if a helping hand is needed, I'll gladly lend one or two. :)

Besides that, the bug could be seen as twofold: Neither should auto-complete.el try to create a popup w/ no contents, nor should popup.el get in an undefined state if someone tries something like that.