Bug 28472: handle items with NULL shelving location

Bug 28472: handle items with NULL shelving location

The UpdateItemLocationOnCheckin System Preference can be set to update the
location of ALL items during check in, regardless of their shelving location.
However, this does not currently work 100% as it is excluding items with no
shelving location (i.e. value of NULL in the corresponding database field).

This patch, based on the comment made by Nick Clemens, fixes that.

Test plan (based on the original Bug Description by Andrew Fuerste-Henry):

1) Have a shelving location CART
2) In UpdateItemLocationOnCheckin, enter "_ALL_: CART" (without the quotes)
3) Check in an item that has a shelving location, confirm it changes to CART
4) Check in an item with a NULL shelving location, confirm it doesn't go to CART
5) Apply this patch
6) Repeat step 4): this time the item should move to the CART shelving location

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>