Before this commit, and since the new orm, we cannot open a record that have
no res_model and res_id.

With new orm, we should return a value for all records.
Now, we can open attachement form view without the error:

    ir.attachement(<id>).res_name traceback

closes odoo/odoo#36446

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>