"Couldn't find User without an ID" Sidekiq::Extensions::DelayedMailer loop

Slightly related to #2758 (closed) ...

A failed Sidekiq job gets stuck in an infinite retry loop when reassigning an issue/MR to someone when no one was previously assigned.

Steps to reproduce in 4.2 stable:

1. Create an issue or merge request without assigning it to anyone
2. Go to issue/MR and click Edit
3. Change the assigned user to someone and save
4. Notice failed job in Sidekiq -> Retries because it's trying to send an email to the previously assigned user (which was nobody)