ibusManager: Don't pass undefined callback to ibus

Since commit 551e827841, we don't always pass a callback parameter.
However passing it on as undefined to ibus doesn't work, as gjs doesn't
accept that as a valid callback value and throw an error. As a result,
we can end up with no layout selected in the keyboard menu and an "empty"
indicator. Fix this by explicitly passing null if no callback has been
provided.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/293

js/misc/ibusManager.js

@@ -210,7 +210,7 @@ var IBusManager = new Lang.Class({
         }
 
         this._ibus.set_global_engine_async(id, this._MAX_INPUT_SOURCE_ACTIVATION_TIME,
-                                           null, callback);
+                                           null, callback || null);
     },
 
     preloadEngines(ids) {