[FIX] event_crm: correctly compare IDs to check registration changes

When checking updated values of a registration to propagate to its linked lead
we may compare a recordset to IDs. This commit fixes that by always comparing
IDs.

Task-3431124

Part-of: odoo/odoo#128823
Co-authored-by: Jeremy Hennecart <jeh@odoo.com>

addons/event_crm/models/event_registration.py

@@ -336,8 +336,8 @@ class EventRegistration(models.Model):
 
     def _convert_value(self, value, field_name):
         """ Small tool because convert_to_write is touchy """
-        if value and self._fields[field_name].type in ['many2many', 'one2many']:
+        if isinstance(value, models.BaseModel) and self._fields[field_name].type in ['many2many', 'one2many']:
             return value.ids
-        if value and self._fields[field_name].type == 'many2one':
+        if isinstance(value, models.BaseModel) and self._fields[field_name].type == 'many2one':
             return value.id
         return value