[FIX] project: fix default assigned to for rating in task

Before this commit, the default assignee set to a rating in a task is
always empty because we check if the `project.task` has a `user_id`
field to give the partner of this user.
Since it is no longer the case because we can recently assign many users
to a task. That is, it is `user_ids` field and no longer `user_id`
field.

This commit overwrites `rating_get_rated_partner_id` method to set by
default the partner of the user contained in `user_ids` field if this
field contains at most one user. Otherwise, we set no partner. We choose
to not select the first user in the user_ids field because we cannot
consider it is the responsible of the task and not another one.
We cannot create a rating per user because the rating average will no
longer right because at the beginning of the task we could have 5 users
and after we could have more users or less users and so the number of
ratings for this task could be different each time we send a rating
review to the customer.

Part of task-2696911

closes odoo/odoo#81009

X-original-commit: 88e7d270bee962d6e4d40f6b62d25a9519abfcf2
Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>
Signed-off-by: Xavier <xbo@odoo.com>

addons/project/models/project.py

@@ -2190,6 +2190,11 @@ class Task(models.Model):
     def _rating_get_parent_field_name(self):
         return 'project_id'
 
+    def rating_get_rated_partner_id(self):
+        """ Overwrite since we have user_ids and not user_id """
+        tasks_with_one_user = self.filtered(lambda task: len(task.user_ids) == 1 and task.user_ids.partner_id)
+        return tasks_with_one_user.user_ids.partner_id or self.env['res.partner']
+
     # ---------------------------------------------------
     # Privacy
     # ---------------------------------------------------