notificationDaemon.js: don't call setApp() if app couldn't be determined

Notify() was calling setApp() unconditionally after attempting to
determine an application. However, determining the application can
fail (for example, when notify-send is used) and resulted in an exception
being printed.

js/ui/notificationDaemon.js

@@ -190,6 +190,7 @@ NotificationDaemon.prototype = {
             let busProxy = new Bus();
             busProxy.GetConnectionUnixProcessIDRemote(sender, function (result, excp) {
                 let app = Shell.WindowTracker.get_default().get_app_from_pid(result);
+                if (app)
                     source.setApp(app);
             });
         } else {