Fix "To: Unknown Object" on outbound Maniphest mail with no owner

Summary: Fixes T7778. This was likely caused by removing an array_filter() somewhere in the course of T7731, but I'd rather have the code be more correct.

Test Plan:
Sent mail on a task with no owner.

• Before patch: unknown recipient.
• After patch: expected recipients.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T7778

Differential Revision: https://secure.phabricator.com/D12320