trivial: Don't show (null) in the UI when the DFU status is unknown
trivial: Don't show (null) in the UI when the DFU status is unknown

fwupd/fwupd,fwupd/fwupd,superm1/fwupd,fwupd/fwupd,vathpela/fwupd,hughsie/fwupd,hughsie/fwupd,vathpela/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,superm1/fwupd,hughsie/fwupd