Don't error out when CollectionModifyJob is simply sending "VIRTUAL 0".

If the collection isn't virtual, this isn't a change.
CollectionModifyJob is simply setting all existing attributes back.
This fixes CollectionModifyJob, which fixes the imap resource not being
able to store the uidnext value, so it was constantly re-listing folders.
(scary, when your inbox disappears, before it reappears...)