belongs_to will now trigger a validation error by default if the association is not present (rails/rails#18937).

This breaks the possibility of creating a Notification without targets, such as:

Notification.create(notify_type: 'follow', actor: self, user: user)

This can be fixed either by adding optional: true in the Model (this PR), or changing Rails.application.config.active_record.belongs_to_required_by_default to false in dev's apps.