Don't look for an attribute on None object

[gamboz]

Not particularly important, but I thought I'd report 🙂

In the decorator article_stage_accepted_or_later_or_staff_required, there is an if clause that might try to access an attribute of a None object.

To replicate, one should log-in to a journal and manually enter the URL of a non existing article in order to trigger the view "article_file_download" (views.serve_article_file).

E.g. https://olh.openlibhums.org/article/id/20000/file/1234/ (ATM article 20000 does not exist).

HTH