Bug 20241: Fix display of publication year in subscription record search for MARC21

Bug 20241: Fix display of publication year in subscription record search for MARC21

The old problem: UNIMARC uses biblio.publicationyear, MARC21 uses
biblio.copyrightdate. Patch adds a fallback, if there is nothing
in publicationyear, copyrightdate will be displayed.

To test:
  - In a MARC21 database
  - Add a new subscription
  - Search for records
  - Verify the publication year column remains empty
  - Apply patch
  - Repeat search
  - Verify that the year now shows

https://bugs.koha-community.org/show_bug.cgi?id=20421

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>