Bug 16114: Koha::ItemType->translated_description should return a string

Bug 16114: Koha::ItemType->translated_description should return a string

This patch fixes a regression introduce by bug 14828.
If an itemtype is translated, its description won't be displayed
correctly on the item search form.

Koha::ItemType->translated_description is only used once.

Test plan:
- define a translation for an item type (Administration ->
  Item types -> Edit -> Translate into other languages)
- go on the item search form, the item type should be displayed
  correctly.

QA note: This module is not covered by tests, it seems that I forgot to
write them...

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com