|To answer my own question:|
It turns out there really is an error in Atom Builder: contributor is not actually optional: I got the error mentioned in my original posting because I was supplying a value for the entry contributor but not feed contributor. When I added a contributor for feed the error vanished.
I haven't tried all permutations of this--i.e. adding a feed contributor but not an entry contributor--but looking at the code, my guess is that contributor is not being treated as an option by atom builder: if you don't have it, you get this error.