[FIX] fields: ondelete being None on many2many field

The issue is simply creating a related stored many2many field, which is
pretty easy to do with Studio.  When creating the field, Odoo crashes
with a traceback caused by ondelete being None on the field.

The source of the bug is the fact that the attribute field.ondelete is
set to a sensible default ('cascade') on non-related fields only.  The
fix consists in setting the default value on the attribute itself, so
that the setup of the field never falls on a case where field.ondelete
is unset.

closes odoo/odoo#86136

Signed-off-by: Raphael Collet <rco@odoo.com>