getRootFolder should not setup the FS for any user

Fixes #22467

This can go wrong when an app (take the ldap app) DIs something that
needs the rootFolder. This break if we use cookie auth since then we
know the user at that point and thus try to setup the fs for that user.

However if there are then incomming shares from an ldap user they will
fails since the user manager can't find them yet.

Now getRootFolder does not setup the fs for any user.