[FIX] sale: sales order email template without portal_sale
[FIX] sale: sales order email template without portal_sale

When disabling the customer portal
in the general settings
(or uninstalling the `portal_sale` module directly),
the model `sale.order` no longer has a method
`get_signup_url`, and it therefore leads
to the fail of the email template rendering which is
still referencing this method.

Besides, the `access_url` is actually used only if
`is_online` is True,
(see the `% if is_online:` few lines later)
and in this case `get_signup_url`
was not used at all.

Therefore, we can assume `None` is a good alternative,
as the resul of `get_signup_url` was actually no longer
used.

opw-710481

hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo