How to explain missing grade on transcript
If the suggestion of Andreas Blass doesn't work for you, then include a note somewhere saying that the professor is late in assigning grades to the class as a whole and that questions about that should be directed to the chair or the dean. Give an email address for that person as well as the name. 

You don't need to accuse the professor of anything and there are several valid reasons for grades being late, whether they apply here or not. But the administration can verify your claims and should be willing to do so. 

Just state the facts and point to an authority who can verify them.