Major Description
Confluence cloud seems to use the allSubjectUsers property more than what we've seen so far. We used to see this property set to empty or to anonymous-users (which we already migrate to XWikiGuest).
There are things like all-licensed-users and all-product-admins now in the allUsersSubject property of space permissions objects.
Unfortunately, confluence-xml currently migrates these space permissions with empty users and groups, giving rights to XWikiGuest.
We ought to interpret what is inside allSubjectUsers like a group (except for anonymous-users, which we'll continue migrating as XWikiGuest).