gdbus-codegen: Fix crasher in goa-using apps

When replacing a version of goa-daemon (from gnome-online-accounts)
by a newer version with some added interfaces, evolution-data-server
and the gvfs-goa volume monitor might crash as there's no interface
definition for this new interface.

Work-around this by returning earlier from the _notify() implementation,
rather than accessing invalid memory.

https://bugzilla.gnome.org/show_bug.cgi?id=720539