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.com/D13245