Do not fail when some result are void in a group.
Do not fail when some result are void in a group.

    * share/urbi/group.u (Group.fallback): Use void-tolerant list insertion
    method.

aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi