[FIX] account: negative bank statement amount

- Create a bank statement for a negative amount, e.g. -100.
- Click 'Reconcile'.
- Create a counterpart line for this bank statement, but specify
  receivable account.

The payment created has `partner_type` set as `supplier`.

Although a receivable account is used for customers.

This use case can arise in case of a credit note to a customer: the
reimbursement should create a customer payment, not a supplier payment.

Co-authored-by: Nicolas Martinelli <nim@odoo.com>

opw-1910807

closes odoo/odoo#30232