[FIX] mass_mailing: fix send with batch

This commit fixes a behavior when we send a batch of mailings
with no recipients.
When there is no recipients passed down, each mailing should
use it's own recipients and not the recipients of the first mailing.

closes odoo/odoo#75389

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>