Bug 33188: Remove warning from Koha::Item->hidden_in_opac

Bug 33188: Remove warning from Koha::Item->hidden_in_opac

"Use of uninitialized value in string eq"

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

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Edit: tidied the tests (tcohen)
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit f575d4cd73567d5a0e0edc777f83bdf99959a526)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 1b27067e9adc695662c570b411bc546c6c7c41c0)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>