Bug 22006: Handle undefined itemnumber for Koha::Account::Line->item

Bug 22006: Handle undefined itemnumber for Koha::Account::Line->item

If there is no item linked to an account line, the ->item method should return undef.
Without this patch it explodes with:
Carp::croak('DBIC result _type  isn\'t of the _type Item') called at /home/vagrant/kohaclone/Koha/Object.pm line 102

Exists since the introduction of this method by bug 12001

Reported on bug 19489 comment 18.

Test plan:
  prove t/db_dependent/Koha/Account/Lines.t
must return green

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>