[FIX] google_calendar: fix attendee deletion

When an attendee is deleted in google, we need to sync the odoo attendees and delete the ones that do not exist anymore in google.
Unfortunately, by doing so, we introduced an issue. We filtered attendees that were no longer existing when we looped on the attendee emails.

```py
odoo.exceptions.MissingError: Record does not exist or has been deleted.
(Record: calendar.attendee(YYY,), User: XXX)
```

This commit ensure that we filter only existing attendees.

X-original-commit: 0fb1002c9bb027d1694b834c52e4492a03c8a09d
Part-of: odoo/odoo#83040

addons/google_calendar/models/calendar_recurrence_rule.py

@@ -109,7 +109,7 @@ class RecurrenceRule(models.Model):
         for odoo_attendee_email in set(existing_attendees.mapped('email')):
             # Remove old attendees
             if email_normalize(odoo_attendee_email) not in emails:
-                attendee = existing_attendees.filtered(lambda att: att.email == email_normalize(odoo_attendee_email))
+                attendee = existing_attendees.exists().filtered(lambda att: att.email == email_normalize(odoo_attendee_email))
                 self.calendar_event_ids.write({'need_sync': False, 'partner_ids': [(3, attendee.partner_id.id)]})
 
         # Update the recurrence values