fu-udev-device: Don't assume all devices have `device_file`
fu-udev-device: Don't assume all devices have `device_file`

Corrects an error during probe for intel graphics:
```
failed to add udev device /sys/devices/pci0000:00/0000:00:02.0: failed to open (null): Bad address
```

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