Bug 23463: (follow-up 3) Fix timestamp default value
This is the only situation I found where:
* t/db_dependent/Koha/Item.t is passing
* t/db_dependent/Koha/Object.t is passing
* MySQL 8 is happy (and not fail with "Column 'timestamp' cannot be null"), which is certainly what I missed in the previous follow-up
About the change to Object.t: the next store was called without the
generated timestamp, so it is needed to call discard_changes.
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>