[ShellAppMonitor] Handle Iceweasel tripping Firefox window title detection

We have compatibility code which detects from the window title what
an application is.  However, the code didn't handle the case where
we discovered by title, but didn't have the expected .desktop file
installed.

src/shell-app-monitor.c

@@ -437,9 +437,10 @@ get_app_for_window_direct (MetaWindow  *window)
 
       if (id != NULL)
         result = shell_app_system_load_from_desktop_file (appsys, id, NULL);
-      else
-        result = create_transient_app_for_window (window);
     }
+  if (result == NULL)
+    result = create_transient_app_for_window (window);
+
   return result;
 }