Handle None amount in FIASPay migration
Handle None amount in FIASPay migration

Production doesn't seem to have such values, but my test system had one
in a test event. So I guess there's some bug in FIASpay that can cause
this and thuse it's better to handle it just in case.

ThiefMaster/indico,OmeGak/indico,mvidalgarcia/indico,ThiefMaster/indico,DirkHoffmann/indico,indico/indico,mic4ael/indico,pferreir/indico,mic4ael/indico,mvidalgarcia/indico,mic4ael/indico,pferreir/indico,OmeGak/indico,pferreir/indico,indico/indico,DirkHoffmann/indico,OmeGak/indico,mvidalgarcia/indico,ThiefMaster/indico,pferreir/indico,indico/indico,indico/indico,OmeGak/indico,mic4ael/indico,mvidalgarcia/indico,DirkHoffmann/indico,DirkHoffmann/indico,ThiefMaster/indico