strip empty entries from sources on the way in
strip empty entries from sources on the way in

For #709

Sinar/popit,mysociety/popit,mysociety/popit,mysociety/popit,openstate/popit,mysociety/popit,openstate/popit,Sinar/popit,openstate/popit,Sinar/popit,mysociety/popit,openstate/popit,Sinar/popit