[FIX] account_payment: make partner_id required only when draft
[FIX] account_payment: make partner_id required only when draft

When not draft, the partner_id field is readonly and cannot be modified
anyway, but it makes the form invalid, so form buttons cannot be used,
e.g. to access the journal items, etc.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo