Use fdmprinter.json If we have no active machine instead of returning None
Use fdmprinter.json If we have no active machine instead of returning None

This prevents issues where backend creation would end up in an infinite
loop while there was no active machine.

Contributes to CURA-1376

ynotstartups/Wanhao,hmflash/Cura,fieldOfView/Cura,ynotstartups/Wanhao,fieldOfView/Cura,Curahelper/Cura,totalretribution/Cura,Curahelper/Cura,senttech/Cura,senttech/Cura,totalretribution/Cura,hmflash/Cura