Bug 33885: Make Koha::Acq::Order->creator return undef if no creator

Bug 33885: Make Koha::Acq::Order->creator return undef if no creator

If the account of the creator of an order has been deleted we should
return undef here, instead of crashing with
GET /api/v1/acquisitions/orders: unhandled exception (Mojo::Exception)<<DBIC result _type  isn't of the _type Borrower at /kohadevbox/koha/Koha/Objects.pm line 445.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 1c3a35418ea3defb50debbab4fd334bee4eb5229)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>