[FIX] account: allow sending invoice to partner other than the customer

When sending an invoice to a recipient who is not the customer, they are
unable to view the invoice in the customer portal and are prompted to
log in.

1. Create and validate an invoice
2. Click on the Send & Print button, then add a recipient who is not the
   customer associated with the invoice.
3. Proceed to send the invoice.
4. Access the email that was sent to the added recipient (who is not the
   customer)
5. Using an incognito or private browsing window, open the link
   `View Invoice`

=> you should see that you are asked to log in, instead of being
directed to the customer portal.

opw-3114579

closes odoo/odoo#121572

X-original-commit: 3676692cb916bfcd417b7c06df6131e6bceb1659
Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>
Signed-off-by: Nshimiyimana Serge Séna (sesn) <sesn@odoo.com>