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>

Koha/UI/Form/Builder/Item.pm

@@ -119,7 +119,7 @@ sub generate_subfield_form {
       $subfield->{kohafield} || 'items.more_subfields_xml';
 
     if ( $prefill_with_default_values && ( !defined($value) || $value eq '' ) ) {
-        $value = $subfield->{defaultvalue};
+        $value = $subfield->{defaultvalue} if !$item->{itemnumber}; # apply defaultvalue only to new items
         if ($value) {
 
 # get today date & replace <<YYYY>>, <<YY>>, <<MM>>, <<DD>> if provided in the default value