[FIX] account: assign current date in due_date if date_ref not available

Unsupported operand type(s) for +: 'NoneType' and 'relativedelta'
occur when we create payment terms without a preview date (date_ref).
This is because date_ref has no value.

This commit solves the issue by adding a condition if date_ref is not
available, so it adds current date instead of it.

sentry-4068322613

closes odoo/odoo#118027

Signed-off-by: William André (wan) <wan@odoo.com>