[FIX] account: sending several invoices at one

When sending several invoices, customers also receive those that do not
concern them.

To reproduce the error:
(Need mailcatcher)
1. Go to Invoicing
2. Select two invoices
    - Status must be "Posted"
    - Customers must be different
3. Action > Send & Print
4. Uncheck "Print", then click on "Send"
5. Repeat 2-4

Err: Each customer receives both invoices. (In some cases, a third mail
is event sent). Each customer must receive his own invoices.

The mail composer wasn't correctly configured.

OPW-2422117

closes odoo/odoo#64314

Signed-off-by: Adrien Widart <adwid@users.noreply.github.com>