Bug 24398: Fix 500 when viewing a single news item at the OPAC

Bug 24398: Fix 500 when viewing a single news item at the OPAC

Test plan:
0. Set the NewsAuthorDisplay preference to 'OPAC' or 'Both OPAC and staff client.'
1. Open the OPAC main page and click on an individual news item
Without this patch you get an ugly 500
    Template process failed: undef error - The method Koha::NewsItem->author_title is not covered by tests!
With this patch applied you see the news with the author's info

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>