Bug running migration "Fix user names"

When you upgrade to the latest version and the migration is added and executed FixUserNames (https://github.com/decidim/decidim/blob/0.18-stable/decidim-core/db/migrate/20190412131728_fix_user_names.rb), it fails because there are users who are "deleted". No email, no name, no nickname, and deleted_at is present. Issue related: https://github.com/decidim/decidim/issues/5191