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 <firstname.lastname@example.org>