[FIX] mail: onchange activity

- Set a default value for the field 'Activity Type'
- Schedule an activity

The default value doesn't appear.

The default value is taken into account, but the onchange override the
value.

opw-1838066

addons/mail/models/mail_activity.py

@@ -140,7 +140,8 @@ class MailActivity(models.Model):
 
     @api.onchange('recommended_activity_type_id')
     def _onchange_recommended_activity_type_id(self):
-        self.activity_type_id = self.recommended_activity_type_id
+        if self.recommended_activity_type_id:
+            self.activity_type_id = self.recommended_activity_type_id
 
     @api.multi
     def _check_access(self, operation):