group: Use proper group constructor
Otherwise we might en up having a group which is not backed by any asset leading to possible assertion as this should never happen (see https://gitlab.gnome.org/GNOME/pitivi/-/issues/2526)
Otherwise we might en up having a group which is not backed by any asset leading to possible assertion as this should never happen (see https://gitlab.gnome.org/GNOME/pitivi/-/issues/2526)