When generating a VAT return with an unclaimed tax amount carried to the
next month, the carryover reimbursement move amounts are computed with
an incorrect ratio.

Steps to reproduce:
- Create and post a bill in May containing a VAT amount.
- Create and post a bill in June containing a VAT amount.
- Create a VAT return for May to carry over the VAT amount to the next
  month.
- Create a VAT return for June, requesting the full VAT amount to be
  reimbursed.
- Validate and send the June VAT return.
- Check the generated reimbursement move

Issue: Line values does not correspond to anything real/tangible.  It
occurs because when computing the ratio for the move we check the last
tax report entry, where we find the amount of tax from the past months
and a line balancing the last month that should not be taken into
account.  The "Balance tax current account (receivable)" line from the
tax closing entry is mistakenly picked up as a tax carried forward line,
throwing off the amounts.

opw-5961836

Forward-Port-Of: odoo/enterprise#115451

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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Forward-Port-Of: odoo/odoo#261039