Udev monitor monitors the whole input subsystem, but not all devices in the input subsystem have devnodes associated to them.
This MR makes the event processing ignore such devices.
All device handling assumes devices have devnodes, so here we just simply ignore all devices which do not have one. They are irrelevant.
Previously, udev bus thread aborted when an event for a device without a devnode was processed:
``` 10111.330:0068:0084:trace:hid:process_monitor_event Received action "remove" for udev device (null) 10111.330:0068:0084:warn:hid:bus_main_thread L"UDEV" bus wait returned status 0xc0000005 ```
Just plugging in and out a normal mouse was enough cause this.
This was because root input devices (which do not have devnodes) were handled too and `find_device_from_devnode()` choked on NULL argument.