Bug 26516: Don't accept incorrect values for copyrightdate/publicationyear

Bug 26516: Don't accept incorrect values for copyrightdate/publicationyear

The previous patch rejects incorrect values when saving to the db specifically for copyrightdate
An error is thrown for the int conversion when it fails

Rather than catching things when saving we can fix the value when generated, simply returning null when parsing
the record

To test:
1 - Add a new record to Koha making sure data is valid except 260$c:
    198-
2 - Save the record
3 - It fails
4 - Apply this patch
5 - Restart all the thigns
6 - Repeat
7 - Success!

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>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>