Bug 35044: (QA follow-up): Allow for value '0' in additional fields

Bug 35044: (QA follow-up): Allow for value '0' in additional fields

This now allows for value '0' for an additional field (repeatable or
not).
This still does not allow for an empty '' field in additional fields.
This is consistent with borrower patron attributes, i.e. if you submit
an empty borrower patron attribute entry, it does not store it as empty.
Additionally, if you have a value in a borrower patron attribute but
then submit the form with that field as empty, it gets deleted from the
database.

I agree not being able to save the value '0' is a blocker, but not the
empty.
For consistency sake I'm keeping this as is, I don't see a use-case
where a row with an empty field value is preferred over it not existing.

If we allow for '' values of additional fields, the 'clear' button for
repeatable fields would have to remove the input instead of clearing the
value (or else how would the user remove repeatable fields entries?).
If this is done, it's inconsistent with the repeatable fields from
patron attribute types, as in that instance the 'clear' button only
removes the input contents, not the input element itself.

prove t/db_dependent/Koha/Object/Mixin/AdditionalFields.t

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>