so test for 'payment' being an instance of 'IZUGFeRDTradeSettlementDebit' as
'IZUGFeRDTradeSettlement' and 'IZUGFeRDTradeSettlementPayment' return 'null' for
'getPaymentXML()' which is not allowed to be written to the XML file.
or a NullPointerException might happen as 'hasDueDate==true' is not the result
of a test to ensure that 'trans' overwrites 'getDueDate()' at all, so the default
implementation from 'IZUGFeRDExportableTransaction' might get used which returns
the unusable result 'null'.
