osdWindow: Fix setting a zero-level in osdWindow

Commit 7101cc3170 caused a small
regression insomuch that it checks for a valid "level" but simply using
"if (level)" which will be false if level is undefined and if
level == 0.

Check for not undefined instead.

https://bugzilla.gnome.org/show_bug.cgi?id=727384

js/ui/osdWindow.js

@@ -125,7 +125,7 @@ const OsdWindow = new Lang.Class({
 
     setLevel: function(level) {
         this._level.actor.visible = (level != undefined);
-        if (level) {
+        if (level != undefined) {
             if (this.actor.visible)
                 Tweener.addTween(this._level,
                                  { level: level,