[FIX] fields: empty date or datetime
[FIX] fields: empty date or datetime

When the `create` or `write` method receives an empty string for a date
or a datetime field, PostgreSQL will fail since this is not an accepted
value for this field type.

We fallback on `None` for falsy values.

opw-1819336

@KangOl : watch out when forward-porting, the signature of
`convert_to_column` has changed in saas-14.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo