Bug 23463: Use new method Koha::Object->set_or_blank

Bug 23463: Use new method Koha::Object->set_or_blank

This patch fixes an issue when editing items.

* The issue
Cannot blank a subfield when editing an item.
If you have an item with itemcallnumber=42, then edit it, blank it and
save. The itemcallnumber is still 42

* Why? (line numbers from https://gitlab.com/joubu/Koha/-/tree/bug_23463)
additem (and other item's edition forms) receives a list of tags,
subfields and values, generates a MARC::Record::XML then calls
ModItemFromMarc:

717     my $itemtosave=MARC::Record::new_from_xml($xml, 'UTF-8');
727         my $newitem = ModItemFromMarc($itemtosave, $biblionumber,
$itemnumber);

And ModItemFromMarc:
 282     my $item = TransformMarcToKoha( $localitemmarc,
$frameworkcode, 'items' );
 283     $item->{cn_source} = delete $item->{'items.cn_source'}; #
Because of C4::Biblio::_disambiguate
 284     $item_object->set($item);

ModItemFromMarc never knows that the field has been blank.
Prior to bug 23463 we had a map of default values, and ModItemFromMarc
was doing:
 426     my $item = TransformMarcToKoha( $localitemmarc,
$frameworkcode, 'items' );
 427     my $default_values = _build_default_values_for_mod_marc();
 428     foreach my $item_field ( keys %$default_values ) {
 429         $item->{$item_field} = $default_values->{$item_field}
 430           unless exists $item->{$item_field};
 431     }

I do not want to reinsert that list of default values.

Here I wrote a generic method in Koha::Object to set the value passed in parameter,
or "blank" if not passed.
It's nulled if can be set to null in DB, or the default value is retrieved from
the schema info.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>