fix(accounts): set payment term outstanding when invoice portion is zero - #59712

Issue

When a Payment Schedule row is added manually with Invoice Portion left at 0 and a Payment Amount entered, outstanding is saved as 0 while payment_amount and base_outstanding hold the full amount. Payment Entry skips terms whose outstanding is below 0.1 when allocating against payment terms, so such rows can never be allocated.

Steps to replicate

1. Create a Purchase Invoice (or Sales Invoice) with no Payment Terms Template.
2. In Payment Schedule, add one row. Set Due Date and Payment Amount to the grand total. Leave Invoice Portion at 0.
3. Save and submit.
4. Compare the row's outstanding with payment_amount and with the invoice outstanding_amount.

Current behaviour: outstanding is 0. payment_amount and base_outstanding equal the grand total, and the invoice outstanding_amount is also the grand total with paid_amount 0.

Expected behaviour: outstanding equals payment_amount, the same way it does when Invoice Portion is set.

Ref: #79701