Bug 32781: Prevent import from list to fail if package does not exist

Bug 32781: Prevent import from list to fail if package does not exist

This is theorical (caught when writting tests) but we need to deal with
it.

Can't call method "resources" on an undefined value at /kohadevbox/koha/Koha/BackgroundJob/CreateEHoldingsFromBiblios.pm line 94.

Test plan:
  prove t/db_dependent/Koha/BackgroundJob/CreateEHoldingsFromBiblios.t
must return green

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>