R1:a06618f87966

Fix an issue where "Send an email to..." rules might be discarded

Summary: Fixes T8464. We could lose the additional users from "Send an email..." rules //if// Herald did not apply any other transactions to the task.

Test Plan:
- Destroyed all Herald rules.
- Created a single "Send an email to..." rule.
- Created a task.
- Saw target get an email.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8464

Differential Revision: https://secure.phabricator.