Bug 25086: Set changed_fields column as nullable

Bug 25086: Set changed_fields column as nullable

This patch changed_fields column as nullable.

To test:
1. Do not apply any patch
2. Set PatronSelfRegistrationVerifyByEmail preference to require
3. Fill self registration form in opac and submit
CHECK => OPAC dies with a nasty message, that says Field 'changed_fields' doesn't have a default value
4. Apply this patch and updatedatabase
5. Repeat step 3
SUCCESS => Registry process continues as expected
6. Sign off

Sponsored-by: Orex Digital
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>