added entry points empty method

@fosdev - I would like to add this a `empty?` method onto the `EntryPoints` collection object.  Wormhole needs this because it has to manage it's expiry manually, because it cannot use Critchon's discovery middleware.  (The discovery middleware is instantiated on application.rb startup, not even sure its application.rb has access to ActiveRecord at that point, so it wouldn't be able to get the ActiveRecord objects that Wormhole uses to build it's entry points.  Even if it could, it would mean that Wormhole would need to be rebooted to get new entry points, created in the database while the app is running).


Any chance @entry_point_objects will be `nil`? If so, should we do something paranoid in the constructor too?