[FIX] base: compute name for all ir_attachments

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>

odoo/addons/base/models/ir_attachment.py

@@ -41,6 +41,8 @@ class IrAttachment(models.Model):
             if attachment.res_model and attachment.res_id:
                 record = self.env[attachment.res_model].browse(attachment.res_id)
                 attachment.res_name = record.display_name
+            else:
+                attachment.res_name = False
 
     @api.model
     def _storage(self):