When someone tries to publish something and there is no mapping present, all they see is an empty "Items to publish" list. My first thought was that it would be helpful to show a message like "You can't publish from this publication because there is no mapping to a web site. Either it's your fault for trying something that shouldn't have worked, or you should find someone that can accept the technical blame for this and berate them severely."
My second thought was that perhaps the existence of a mapping could be taken in to consideration when calculating whether to allow a publish action in the first place. Even that's not ideal, because it doesn't give any hint as to the solution either. So please do something better than either of my ideas.