[FIX] account_voucher: avoid error if no partner

The field partner_id is not required on an account.voucher but the validation was failing if none was set (opw 611663).
This patch makes a fallback on the account of the voucher if neither a partner nor a writeoff account is specified.
[IMP] account_voucher: make the field tax_amount always readonly

The field tax_amount is fieled with onchanges and the compute_tax method. Setting a different value than the one computed by the system may lead to unbalanced move (which is obviously wrong).
In the future, handeling these operations by setting the correct value to the tax accounts would be better.