Avoid returning null from register call, clean up empty registrations. Part of #217
Avoid returning null from register call, clean up empty registrations. Part of #217

martinthomson/ServiceWorker,martinthomson/ServiceWorker,justinleoye/ServiceWorker,zuzusik/ServiceWorker,karolklp/ServiceWorker,karolklp/ServiceWorker,karolklp/ServiceWorker,justinleoye/ServiceWorker,zuzusik/ServiceWorker,zuzusik/ServiceWorker,mkruisselbrink/ServiceWorker,mkruisselbrink/ServiceWorker,chadoliver/ServiceWorker,slightlyoff/ServiceWorker,chadoliver/ServiceWorker,justinleoye/ServiceWorker,chadoliver/ServiceWorker,martinthomson/ServiceWorker,slightlyoff/ServiceWorker