On server side you can use a custom ServiceLayerDecorator and implement the validate method to customize the validation process (ReflectiveServiceLayer contains the default way of validaton).
In your bean you could use validation groups: the default one that is used by the client side code and an additional server validation group. In your ServiceLayerDecorator you would then validate against the server validation group. I think that way you should be able to allow null fields on client side but disallow them on server side.
-- J.