trivial: fu-device: don't show messages with (null) for device ID
trivial: fu-device: don't show messages with (null) for device ID

device ID might not yet be initialized, so just show the word
"unknown"

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