If I try to sale item(s) with zero value, no 'add payment' button appears, but 'complete' button appears directly and sale can be completed. Is it intentional or a bug ?
The sale receipt looks like this...

This is expected behavior. No more paymente can be added once payment due equals 0. In this case you can just complete the sale.
Works as per design.
Is there any usecase that would justify changing this behavior?
Guys, let's not look for what nobody will ever do....