[FIX] event: send event mails for registration without date_open

date_open is not a required field of event.registration.
There is therefore very likely that this field could
not be defined. We should therefore not rely on the fact
it is always defined.

When trying to sent an event mail for an event registration
with a date_open not set (false), the email wasn't sent,
and it prevented the rest of the event mail queue to be
sent as well.

opw-632549

addons/event/event_mail.py

@@ -110,6 +110,8 @@ class EventMailRegistration(models.Model):
     @api.depends('registration_id', 'scheduler_id.interval_unit', 'scheduler_id.interval_type')
     def _compute_scheduled_date(self):
         if self.registration_id:
-            self.scheduled_date = datetime.strptime(self.registration_id.date_open, tools.DEFAULT_SERVER_DATETIME_FORMAT) + _INTERVALS[self.scheduler_id.interval_unit](self.scheduler_id.interval_nbr)
+            date_open = self.registration_id.date_open
+            date_open_datetime = date_open and datetime.strptime(date_open, tools.DEFAULT_SERVER_DATETIME_FORMAT) or fields.datetime.now()
+            self.scheduled_date = date_open_datetime + _INTERVALS[self.scheduler_id.interval_unit](self.scheduler_id.interval_nbr)
         else:
             self.scheduled_date = False