This seems to be the approach taken by Passonneau (1998); her definition of a null Cb seems to imply that a Cb must be (noninferable and) explicitly realized, and her null Cbs constitute the cases in which there is no explicit Cb.
this has no effect on the analysis presented in this article, except for the fact that had we separated these clauses, it would have made the problematic situation we describe later even more marked.