[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.

odoo/fields.py

@@ -1527,6 +1527,9 @@ class Date(Field):
         """ Convert a :class:`date` value into the format expected by the ORM. """
         return value.strftime(DATE_FORMAT) if value else False
 
+    def convert_to_column(self, value, record):
+        return super(Date, self).convert_to_column(value or None, record)
+
     def convert_to_cache(self, value, record, validate=True):
         if not value:
             return False
@@ -1596,6 +1599,9 @@ class Datetime(Field):
         """ Convert a :class:`datetime` value into the format expected by the ORM. """
         return value.strftime(DATETIME_FORMAT) if value else False
 
+    def convert_to_column(self, value, record):
+        return super(Datetime, self).convert_to_column(value or None, record)
+
     def convert_to_cache(self, value, record, validate=True):
         if not value:
             return False