[FIX] event: Set default value for event_count

Fixes https://github.com/odoo/odoo/pull/39583
This commit adds a default value for  event_count
Assigning default value for non-stored compute fields is required in 13.0

closes odoo/odoo#39974

X-original-commit: 9ca72b98f54d7686c0e6019870b40f14dbdd2881
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

addons/event/models/res_partner.py

@@ -9,6 +9,7 @@ class ResPartner(models.Model):
     event_count = fields.Integer("Events", compute='_compute_event_count', help="Number of events the partner has participated.")
 
     def _compute_event_count(self):
+        self.event_count = 0
         if not self.user_has_groups('event.group_event_user'):
             return
         for partner in self: