The special setup for nicely-named groups which have a single
hidden subgroup containing the actual packages, has the problem
that there is a non-empty subgroups item, but this results
in zero actual children: then the number of selected and partly
selected children is also zero in updateSelected() and therefore
the item ends up unselected.
Special-case this to avoid unnecessarily unselecting the item.
Reported by Vitor L.