Bug 32357: Set borrower_message_preferences.days_in_advance default to NULL

Bug 32357: Set borrower_message_preferences.days_in_advance default to NULL

To test:
1. prove t/db_dependent/cronjobs/advance_notices_digest.t
2. prove t/db_dependent/Letters/TemplateToolkit.t
3. prove t/db_dependent/Koha/CurbsidePickups.t
4. prove t/db_dependent/Koha/Patrons.t
5. prove t/db_dependent/Patron/Messaging.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 6871dc7b35f5336503c1c605022ca2d8610af64d)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>