Bug 27714: Prevent Koha::NewsItem->author to explode

Bug 27714: Prevent Koha::NewsItem->author to explode

If the author of a news has been removed, Koha::NewsItem->author must
not explode

DBIC result _type  isn't of the _type Borrower at t/db_dependent/Koha/News.t line 68.

Test plan:
 prove t/db_dependent/Koha/News.t
must return green, and changes must be consistent

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>