(nordigen) fallback to array version of remittanceInformationUnstructured if necessary (#745)

Try falling back to `remittanceInformationUnstructuredArray.join(', ')`
when neither `debtor/creditorName` nor
`remittanceInformationUnstructured` fields are present.

Addresses a suggestion from
https://github.com/actualbudget/actual/issues/724#issuecomment-1464907064