Thank you Raymond.  What I missed before is a) the OP's misnamed EmptyIterator is an iterable (possibly non-empty) but not an iterator, empty or otherwise, and b) a sequence __len__ that lies is just a bug.  (So is a iterator that does not yield the contents of a collection.)  A non-0 length(lst) is a promise that lst[0] exists.  Depending on this is routine.  If json.encoder line 296, 'for value in lst:' were replaced by the following, which should be equivalent,
        for i in range(len(list)):
            value = lst[i]
the encoding would die with IndexError.

The intention that buggy code should not cause a crash was met in this case.