[FIX] website_event: correctly support registrations without ticket

Current code handles "no ticket registration" having ticket_id set to 0
as a valid event.event.ticket ID. We fix that issue by correctly setting
the many2one to False.

Followup of cb928ed613

X-original-commit: 7c1acba32d814ec287c60d0a95f3911d51678932

addons/website_event/controllers/main.py

@@ -303,7 +303,7 @@ class WebsiteEventController(http.Controller):
             if field_name not in registration_fields:
                 continue
             elif isinstance(registration_fields[field_name], (fields.Many2one, fields.Integer)):
-                value = int(value)
+                value = int(value) or False  # 0 is considered as a void many2one aka False
             else:
                 value = value