Rose::DB::Object::ConventionManager::Null is a subclass of Rose::DB::Object::ConventionManager that does nothing. That is, it overrides every method with no-ops that always return undef or an empty list, depending on calling context. This class is a singleton.
When a piece of metadata is missing, the convention manager is asked to provide it. This null convention manager class is useful if you do not want the convention manager to provide any information. In other words, use this convention manager class if you want to force all metadata to be explicitly specified.
See the Rose::DB::Object::ConventionManager documentation for more information on convention managers.