[FIX] mail: do not have _get_message_needaction crash on empty task list

Suppose that a Task is added on a task via a studio customisation.
Then the _get_message_needaction method gets called on the onchange of the
toplevel task, which triggers another call to _get_message_needaction.
During that update, we get a self with a model.newID; its ids field is empty,
so the list of tasks res itself is empty.
As a result the method crashes on a malformed query.
This can in turn prevent the completion of some higher-level operation,
e.g. a state change on the task.

opw 1909043

closes odoo/odoo#29084

addons/mail/models/mail_thread.py

@@ -196,6 +196,8 @@ class MailThread(models.AbstractModel):
     @api.multi
     def _get_message_needaction(self):
         res = dict((res_id, 0) for res_id in self.ids)
+        if not res:
+            return
 
         # search for unread messages, directly in SQL to improve performances
         self._cr.execute(""" SELECT msg.res_id FROM mail_message msg