[FIX] account: partner_id set on a bank fee

Used case:

-Create several customer invoices and validate them
-Register a payment without any partner_id and in a bank statement for an amount
a bit lower than the total of the invoice (the difference is the paypal fees)
-Reconcile the invoices with the payment and create a write-off for the paypal fees
-When you close the bank statement, check the journal items, the paypal fees are
automatically assigned with a partner.

Fix:

-When creating the account move line for the fee, if all the account move lines
linked to the move are for different partners then you cannot determine the partner
of the fee.

opw:674822