Rearding the order, we may be less strict.
The empty modification should be accepted. The server won't do anything. This is a direct consequence of what RFC 4511 says about Modify operation :
"A replace with no value will delete the entire attribute if it exists, and it is ignored if the attribute does not exist."
When the server received a ModifyRequest with a replace without values for a non existing attribute, the attribute is removed from the request. If we don't have any remaining attribute, the server does nothing.
A very special corner case, IMO...