Bug 18539: Forbid list context calls for Koha::Objects->find

Bug 18539: Forbid list context calls for Koha::Objects->find

Reading https://perlmaven.com/how-to-return-undef-from-a-function
this sound like the more correct behaviour.

Considering:
$template->param(
    stuff => Koha::Stuffs->find( $id ),
    foo   => 1,
);
without this patch, if the $id does not represent any rows in the DB,
stuff will be assigned to 'foo' and $foo will be undef in the template.
That can lead to very bad side-effects.

With this patch we make sure that it will never happen again.

Test plan:
  prove t/db_dependent/Koha/Objects.t
should return green

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>