Remove buildpacks with null urls

A failure in the DEA was caused by a nil URL sent by the cloud_controller.  Below is part of the message sent.  The blobstore can return a nil URL if the file is not present.

{"timestamp":1391436356.996865,"message":"nats.message.received","log_level":"debug","source":"Dea::Nats","data":{"subject":"staging.0-22e87f41e72044898fab0b448f67e1bd.start","data":{"app_id":"39c1650d-b1fa-45bd-bf92-bb5cba62545c","task_id":"290b114cbc274bceaa67fc953f1d7b01",

"admin_buildpacks":[{"key":"9871f17b4f441930422b1bbe9262b7513457a22f","url":null},{"key":"94b098dab91e77d2ce048af6193ce5957d0193f6","url":"http://a:b@10.0.116.48:9022/v2/buildpacks/aaeac7a7-a199-47f5-88b6-fe21d898d46f/download"}]

}},"thread_id":20146300,"fiber_id":27311040,"process_id":1420,"file":"/var/vcap/packages/dea_next/lib/dea/nats.rb","lineno":148,"method":"handle_incoming_message"}