[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