Allow for an empty device list during add
This allows for udev-hid-bpf add - path/to/bpf.o [...] at which point we inspect the given BPFs, resolve those to BUS:VID:PID prefixes based on the HID_DEVICE entries and simply resolve those prefixes to any device in /sys/bus/hid/devices/BUS:VID:PID.*.
Especially during development this makes life easier as the sysfs paths can change as devices are unplugged/bpfs are loaded so letting udev-hid-bpf do all the hard work makes life easier.
Closes #40 (closed)