Payment Selection improvements/bugs

In the Payment selection form there are the following bugs to be fixed:

1. Sometimes it generates payments with amount = 0, which is not logical and is inconsistent with the expected behavior.

2. Sometimes the total amount to pay by a business partner is <= 0, in these cases, the bank denies the payment because it generates a negative or zero payment. A new flag on the form and process has been added to show only those business partners with a positive balance.