Fix storage service crash when matching a local contact without an ID.

It's possible that we could match a local contact that doesn't have a
storageId, which would crash when we tried to make a model from it for
merging. This isn't an impossible case -- it could be that the manifest
has record of a user that is newly registered (or just registered at
some point and never deleted) and so we need to give our local record a
storageId for merging.