Bug 27253: (follow-up) Fix UNIXTIME(0) in db_revs/211200037.pl

Bug 27253: (follow-up) Fix UNIXTIME(0) in db_revs/211200037.pl

MariaDB [koha_myclone]> update borrowers set updated_on=COALESCE( NULL, FROM_UNIXTIME(0) ) where borrowernumber=51;
ERROR 1292 (22007): Incorrect datetime value: '1970-01-01 00:00:00' for column `koha_myclone`.`borrowers`.`updated_on` at row 1

MariaDB [koha_myclone]> update borrowers set updated_on=COALESCE( NULL, FROM_UNIXTIME(1) ) where borrowernumber=51;
Query OK, 1 row affected (0.008 sec)
Rows matched: 1  Changed: 1  Warnings: 0

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested by switching updated_on to datetime. Remove NOT NULL, etc.
Copied dbrev to atomicupdate folder.
Resulted in:
    Updated all NULL values of borrowers.updated_on to GREATEST(date_renewed, dateenrolled, lastseen): 51 rows updated

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>