config/udev: consider ID_INPUT_FOO=0 as 'unset'

Historically we didn't need to care about this case but more devices are
having invalid types set and they cannot be unset with a hwdb entry (which
doesn't handle the empty string). Allow for "0" to mean "unset" because
anything else would be crazy anyway.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 5aad8144)