[FIX] calendar: detaching event makes recurring attributes irrelevant

Have a recurring meeting, repeated until some date

Edit one of the instances and detach it (click on "update only this instance")

Edit the detached event and change some value

Save

Before this commit, a JS error was thrown saying that the field final_date was invalid
This was because the required attribute on this field was set to True, but the python
put the value of the final_date to False

After this commit, there is no error as we set this value to false, since the field
doesn't make sense for a non-recurring event anyway

OPW 1892426

closes odoo/odoo#28850

addons/calendar/models/calendar.py

@@ -1371,7 +1371,8 @@ class Meeting(models.Model):
                 rrule_type=False,
                 rrule='',
                 recurrency=False,
-                final_date=datetime.strptime(data.get('start'), DEFAULT_SERVER_DATETIME_FORMAT if data['allday'] else DEFAULT_SERVER_DATETIME_FORMAT) + timedelta(hours=values.get('duration', False) or data.get('duration'))
+                final_date=False,
+                end_type=False
             )
 
             # do not copy the id