Bug 23847: Fix custom item search fields if subfield is '0'

Bug 23847: Fix custom item search fields if subfield is '0'

The subfield is not used if is false, so if 0. We want to test if the
subfield is defined and not an empty string.

Test plan:
- Define a new custom item search fields using a subfield '0'
(withdrawn=952$0 for instance)
- Define one which does not have a subfield (control field)
- Search items using those newly created fields.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>