ManagedAuthResourceOwner<T> class Null safety

REQUIRED: An OAuth 2.0 Resource Owner as a ManagedObject.

An application using this library must declare a ManagedObject subclass that implements this type. This type will represent instances of a resource owner in an application. For example,

    class User extends ManagedObject<_User> implements _User, ManagedAuthResourceOwner  {}
    class _User extends ManagedAuthenticatable { ... }

Note that this interface is made up of both ResourceOwnerTableDefinition and ManagedObject. The type declaring this as an interface must extend ManagedObject and implement a table definition that extends ResourceOwnerTableDefinition. Since all ManagedObject subclasses extend their table definition, this interface requirement is met.