Pass "null" as the argument to a client-only collection.

Previously, a collection created with the `--where "client"` flag would still pass a collection name to `Mongo.Collection` on the client side, leading to "Meteor: insert failed: Method not found" and similar errors.  Difficult to see why you'd pass the `--where "client"` flag if you didn't want the collection to be unmanaged/unsynchronised/local.