Fixed the None quantity bug in dbmanager.undo_bill
Fixed the None quantity bug in dbmanager.undo_bill

Fixes https://github.com/thedos1701/OpenGenfri/issues/28
NonnEmilia/OpenGenfri,radome/OpenGenfri,radome/OpenGenfri,NonnEmilia/OpenGenfri,radome/OpenGenfri,radome/OpenGenfri,NonnEmilia/OpenGenfri,NonnEmilia/OpenGenfri