XWIKI-6275: Removing the last member of a group removes the group as wel...

JIRA: http://jira.xwiki.org/browse/XWIKI-6275

As Denis said:
> When the UI create a new group, the UI put an empty user in the group so it get listed in the group list. If you 
> delete all member of such group, that empty member stay, and the group is kept. Groups imported from the 
> default UI does not have that empty member, and therefore, disappear."

So, I have added an empty user to XWikiAllGroup and XWikiAdminGroup, and now, when I delete XWIki.Admin in a subwiki, the groups are not removed as well.