account: Keep partner_id from counterpart_vals if existing

In case an open balance needs to be created on a receivable or
payable account after reconciliation, but no partner was defined
on the statement line, we want to keep the partner from the
counterpart values that was identified through reference matching.

closes odoo/odoo#86752

X-original-commit: 8c4d6d36
Signed-off-by: Florian Gilbert <flg@odoo.com>