Bug 34620: Do not throw expection if payment type is writeoff

Bug 34620: Do not throw expection if payment type is writeoff

To test:
1. Turn on RequirePaymentType
2. Create a manual invoice and then attempt to write it off.
3. 500 error
4. Turn of RequirePaymentType, no error.
5. Apply patch, restart_all
6. Try step 2 again, you should not get an error
7. prove t/db_dependent/Koha/Account.t
8. Make sure tests pass

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>