rake reports:daily task fails with following trace

rake aborted!
NoMethodError: undefined method `mail_enabled?' for nil:NilClass
/home/ares/Projekty/Zdrojaky/foreman/app/models/user_mail_notification.rb:10:in `deliver'
/home/ares/Projekty/Zdrojaky/foreman/lib/tasks/reports.rake:97:in `block in process_notifications'
/home/ares/Projekty/Zdrojaky/foreman/lib/tasks/reports.rake:96:in `process_notifications'
/home/ares/Projekty/Zdrojaky/foreman/lib/tasks/reports.rake:102:in `block (2 levels) in <top (required)>'
/home/ares/.rbenv/versions/2.2.2/bin/bundle:23:in `load'
/home/ares/.rbenv/versions/2.2.2/bin/bundle:23:in `<main>'
Tasks: TOP => reports:daily
(See full trace by running task with --trace)

it's obviously caused by taxonomies, the rake task is not running under admin user so mail notification user can not be found if the user is in any context. I think that's a candidate for 1.15.z since it's a regression caused by #16982