Fixed #15524 -- Marked up for translation literal shown by the admin app e.g....

Fixed #15524 -- Marked up for translation literal shown by the admin app e.g. in nullable changelist cells and filters. Thanks charettes for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15698 bcc190cf-cafb-0310-a4f2-bffc1f526a37