Clean up Maniphest email "to"

Summary:
We currently try to send Maniphest email "To" the owner and actor, but for unassigned tasks there is no owner.

Just filter the PHIDs in the parent, since it's reasonable for subclasses to be liberal about construction here.

Test Plan: Commented on an unassigned task, got an email without a bogus "To".

Reviewers: btrahan, asherkin

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7129