[Koha] zebra - item-level_itypes errors

[Mark Alexander]

Excerpts from Tom Hanstra's message of 2017-04-19 12:13:30 -0400:
> item-level_itypes set but no itemtype set for item (item#) at
> /usr/share/koha/lib/Koha/Schema/Result/Item.pm line 698

Our installation has item-level_itypes enabled, but all of our item
records (imported from another ILS) have the itype set to a non-NULL
value.  When I manually set the itype of an item to NULL, I see the
above warning in /var/log/koha/INSTANCE/indexer-output.log.