Your request give us an idea.
With last beta, if some diagram is empty, we do not call "dot" anymore (which was useless anyway).
So the following one is working with http://beta.plantuml.net/plantuml.jar out of the box, even without dot.
Furthurmore, we realize that we do not need dot when there is only one class without any link.
So the following example is also working without dot:
Maybe it could be useful for some people.