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)
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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)