window-list: Check if the window title is set

MetaWindow.title might be NULL, particularly for Wayland clients where
setting the title is a request separate from window creation. We
shouldn't try to set StLabel's text prop in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=745064

extensions/window-list/extension.js

@@ -169,6 +169,9 @@ const WindowTitle = new Lang.Class({
     },
 
     _updateTitle: function() {
+        if (!this._metaWindow.title)
+            return;
+
         if (this._metaWindow.minimized)
             this._label.text = '[%s]'.format(this._metaWindow.title);
         else