Bug 19978: Fix ITEMTYPECAT behaviour

Bug 19978: Fix ITEMTYPECAT behaviour

ITEMTYPECAT permits to group and hide item types at the OPAC (see bug
10937 for a complete description).

Since commit 091d6c513bcbee224ff06477e79be48cea7fe825
    Bug 17843: Replace C4::Koha::getitemtypeinfo with Koha::ItemTypes
the code assume that they are item types. Before it just assigned undef
to the description.

Test plan:
Create ITEMTYPECAT authorised values
Assign an item type to this authorised value group
Search for a item using this item type at the OPAC
Without this patch applied you get:
Can't call method "translated_description" on an undefined value at
/home/vagrant/kohaclone/opac/opac-search.pl line 231.

With this patch applied the search result is displayed.

Make sure the original feature still works.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>