allow empty configuration for bundle

[havvg]

Currently it's not possible to have an empty configuration when enabling the bundle.

When running a multi-tenant application, this is a nice way to have the bundle enabled, but only provide configuration (firewalls, resource owners) where actually required. The bundle should just skip everything when no resource owner is configured, effectively disabling itself.

Upon an empty configuration the following exception is thrown.

  [Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException]
  The service "hwi_oauth.security.oauth_utils" has a dependency on a non-existent service "hwi_oauth.resource_ownermap.main".