popupMenu: Hide separator label when not in use

The optional label support introduced in commit af063dc2f2 broke
the centering of separators, as the label still adds additional
spacing even when empty.
Properly hide the label actor in that case to fix the alignment.

https://bugzilla.gnome.org/show_bug.cgi?id=730753

js/ui/popupMenu.js

@@ -251,8 +251,16 @@ const PopupSeparatorMenuItem = new Lang.Class({
         this.actor.add(this.label);
         this.actor.label_actor = this.label;
 
+        this.label.connect('notify::text',
+                           Lang.bind(this, this._syncVisibility));
+        this._syncVisibility();
+
         this._separator = new Separator.HorizontalSeparator({ style_class: 'popup-separator-menu-item' });
         this.actor.add(this._separator.actor, { expand: true });
+    },
+
+    _syncVisibility: function() {
+        this.label.visible = this.label.text != '';
     }
 });