[FIX] account: Unable to add credit note to open invoice

Steps to reproduce the bug:

- Create a Sale Tax T and the field account_id set and let the field refund_account_id empty
- Create a customer invoice CI with a line with T
- Validate CI
- Add a credit note and fill the reason
- Click on 'Add credit note'

Bug:

An error was raised.

opw:2154932

closes odoo/odoo#42488

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>