Fix bug for ranking Qs with non-unique weights

API can sometimes return multiple matrix ranking answers with the same
rank / weight which it probably shouldn't be able to do. Will have to
avoid using a dictionary here since this means that keys are non-unique.
Fix issue when API adds spurious empty answers

Occasionally the API returns spurious extra empty answers with
questions, which it shouldn't. Need to ignore these to prevent data
processing crashing out.