inputMethod: Check return value when letting IM handle virtual keys

When propagating keys from the OSK, we usually feed these directly to
the IBusInputContext and let the IM handle the effects of this virtual
key event (which may also include feeding a key event back to us).

But these functions may also return a FALSE value if the key was "let
through" by the IM, which means the ball is in our yard again, and
we are responsible of letting this event get to its destination.

If that happens, just fall through, so the string is committed to
the client as an UTF-8 string, or propagated through keyboard events.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5930
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2508>