Bug 29146: Same for item editor

Bug 29146: Same for item editor

Test plan:
Put some defaultvalue in an item subfield.
Test adding new item; is defaultvalue supplied already?
Test adding existing item. Clear the subfield. Save. Open again.
Is the subfield still empty now?

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>