End enrollment email notification not sent for rollouts

[data-sync-user]

[Prerequisites]:

• Have an experiment created by another user. 

[Steps to reproduce]:

1. Open the Nimbus page of the experiment.
2. Scroll down to the “Overview” section and click on the Subscribe button
3. Navigate to the Nimbus Experimenter Admin page (+https://stage.experimenter.nonprod.dataops.mozgcp.net/admin/+) and  set the “Proposed Enrollment Duration” to “0”.
4. Save the changes and wait a couple of minutes.
5. Observe the inbox of the subscribed account.

[Expected result]:

• An “End Enrollment” email is sent to both the experiment owner and subscribers.

[Actual result]:

• No email notification is sent.

[Notes]:

The “End Enrollment” experiment emails are correctly sent, only rollouts are affected.

┆Issue is synchronized with this Jira Bug

[freshstrangemusic]

I believe this is intended behaviour (c.f. https://github.com/mozilla/experimenter/pull/8908)