messageTray: Support setImage(null) to mean unsetImage()

This is a quick API change that should clean up some conditionals.

https://bugzilla.gnome.org/show_bug.cgi?id=680414

js/ui/messageTray.js

@@ -611,6 +611,8 @@ const Notification = new Lang.Class({
     setImage: function(image) {
         if (this._imageBin)
             this.unsetImage();
+        if (!image)
+            return;
         this._imageBin = new St.Bin();
         this._imageBin.child = image;
         this._imageBin.opacity = 230;