Failed to send notification mail when unapprove a MR without an assignee

Error in sidekiq.log:

ActionView::Template::Error: No route matches {:action=>"show", :controller=>"users", :username=>nil} missing required keys: [:username]

/opt/gitlab/embedded/service/gitlab-rails/app/views/notify/unapproved_merge_request_email.html.haml:133

Maybe related to this code:

https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/views/notify/unapproved_merge_request_email.html.haml#L133

@merge_request.assignee here could be nil.