The guest user can be returned as a member of a group

When a group page holds a XWiki.XWikiGroups object whose member is the guest user (XWiki.XWikiGuest), GroupManager#getMembers() returns XWiki.XWikiGuest as a member of that group, even though the guest user can't be a member of any group: the rights system never gives the guest user any groups. The same goes for XWiki.superadmin, and an empty member value is returned as the group itself.

Steps to reproduce

1. Add an XWiki.XWikiGroups object with member XWiki.XWikiGuest to XWiki.XWikiAllGroup
2. Call $services.user.group.getMembers($xwiki.getDocument('XWiki.XWikiAllGroup').documentReference, false)

Expected

The guest user is not listed.

Actual

xwiki:XWiki.XWikiGuest is listed as a member.