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
(cherry picked from commit 74bb9e6249)