notification mailer fails since 6.7 upgrade

Created by: Elyahou

I have updated my gitlab+gitlabshell installation yesterday, from 6.6.x to 6.7.2 with the upgrader.

Since, many users dont receive the notification emails for mergerequest.

In the sidekiq panel, I can see 150 failed jobs,

On the retry tabs, I can see the errors:

Arguments:
"---\n- !ruby/class 'Notify'\n- :merged_merge_request_email\n- - 12\n - 255\n"

Error:
ActiveRecord::RecordNotFound: Couldn't find User without an ID