Don't require missing fields be present in order to subscribe to an object

Summary: Fixes T8719. It's fine to subscribe to objects that don't have all their required fields filled in.

Test Plan: Subscribed/unsubscribed from an object.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8719

Differential Revision: https://secure.phabricator.com/D13484