Commit 17d9ec5788fb made the input method call update() more eagerly,
but also at times that it does not have a cursor position yet. Make
it bail out correctly in that situation.
Fixes: 17d9ec5788fb ("inputMethod: Keep Capabilite.FOCUS before context.focus_in/focus_out")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2876>