Error in approved_merge_request_email when MR doesn't have an assignee

https://sentry.gitlap.com/gitlab/gitlabcom/issues/18356/

ActionController::UrlGenerationError: No route matches {:action=>"show", :controller=>"users", :username=>nil} missing required keys: [:username]
  app/views/notify/approved_merge_request_email.html.haml:133:in `_app_views_notify_approved_merge_request_email_html_haml___2150983067848982319_70196705745160'
    %a.muted{ href: user_url(@merge_request.assignee), style: "color:#333333;text-decoration:none;" }
  app/mailers/notify.rb:133:in `mail_thread'
    mail(headers)
  app/mailers/notify.rb:161:in `mail_answer_thread'
    mail_thread(model, headers)
  app/mailers/emails/merge_requests.rb:61:in `approved_merge_request_email'
    mail_answer_thread(@merge_request, merge_request_thread_options(approved_by_user_id, recipient_id))
  lib/gitlab/metrics/instrumentation.rb:155:in `block in approved_merge_request_email'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
...
(121 additional frame(s) were not displayed)

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

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

moved from gitlab-ce#27140

added ~978335 ~481018 ~238305 notifications regression labels

changed milestone to %8.17

Most probably introduced by 4e5512d985d /cc @brycepj

assigned to @rymai

mentioned in merge request !1103 (merged)

@rymai I'm on it -- thanks for the ping.

Edit:

JK - You already did it. Thanks

assigned to @brycepj

assigned to @rymai

@brycepj Hehe yes, it looked easy to fix so...

closed via merge request !1103 (merged)

mentioned in commit 043cf215