Don't throw script errors when ordered before udev
    
    Due to the way libdnf/libsolv resolves RPM installation order, it is
    possible that this package will be installed before systemd-udev is
    installed. If that happens, /sbin/udevadm is unavailable and an error
    is thrown. However, it's safe to skip the call to /sbin/udevadm if
    systemd-udev is not available yet. When that package is installed,
    it runs the necessary actions itself.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>