Bug 21475: Fix crash on missing default itemtype

Bug 21475: Fix crash on missing default itemtype

Test plan:
Enable ArticleRequests.
Find book without itemtype (942c). Maybe you need this:
  update biblioitems set itemtype=NULL where biblionumber=[...]
Goto opac detail for that book. No crash anymore?

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>