portalHelper: Fix runtime warning

When there is no URI, then the title should be empty, not null:
Gjs-WARNING **: JS ERROR: Error: Argument 'str' (type utf8) may not be null

https://bugzilla.gnome.org/show_bug.cgi?id=778552

js/portalHelper/main.js

@@ -176,7 +176,7 @@ const PortalWindow = new Lang.Class({
         if (uri)
             this._headerBar.setSubtitle(GLib.uri_unescape_string(uri, null));
         else
-            this._headerBar.setSubtitle(null);
+            this._headerBar.setSubtitle('');
     },
 
     refresh: function() {