Bug 21336: (follow-up) Handle strict SQL mode in _anonymize_column

Bug 21336: (follow-up) Handle strict SQL mode in _anonymize_column

When a field is not nullable we should pass empty string, zero or today
to a char, numeric or date column.
The mandatory parameter does not refer to a database constraint but a Koha
preference. Only for strings we generate a random value.

Test plan:
Enable strict_sql_modes.
Run t/db_dependent/Koha/Patrons.t again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>