Both the newsletter and the statistics record need to have the sentAt
set explicitly. Fixing that caused the final check for average segment
score to fail because subscriber1 only has 2 sent emails in the past
year, resulting in a null value for engagement score, which doesn't
contribute to the average calculation.
MAILPOET-5508
If someone hasn't received at least 3 emails in the last year, we can't
trust that their level of engagement would be the same now as it was
over a year ago.
MAILPOET-5410
