During a previous fix (https://github.com/odoo/odoo/pull/206158), fallback values were added for an explicit `email_to` if the default email template for the abandonned cart was missing them.

But since (https://github.com/odoo/odoo/pull/172714), the template uses `use_default_to` == True, which will compute the default partner and add them to `partner_ids` of the `mail.mail` record.

So the old bug of "abandoned cart email is sent twice" reappered: the if condition fails to account for `use_default_to` being set, so the partner email is set explicitly on `email_to` AND referenced in `partner_ids`, which de-facto sends the email twice to the customer on the sales order.

## FIX:
We account for `use_default_to` in the if condition before adding the fallback.

How to reproduce:
1) Setup a database with demo data and website_sale 
2) Visit the shop as a visitor, add stuff to your cart 
3) Sign up for portal access and add new stuff to cart (will attribute SO to new portal account)
4) Wait for the abandoned cart email to trigger (can be forced by playing with `cart_recovery_email_sent`: false,`is_abandoned_cart`: true and triggering the CRON) -> mail is sent twice to the customer

### Remarks:
- changed the unit test to actually capture the generated `mail.mail` and apply some asserts on it

OPW-6134731


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