librc: fix rc_runlevel_exists return for empty string
librc: fix rc_runlevel_exists return for empty string

This function should return false if the runlevel is an empty string.

    X-Gentoo-Bug: 803536
    X-Gentoo-Bug-URL: https://bugs.gentoo.org/803536
    Closes: https://github.com/OpenRC/openrc/pull/431

williamh/openrc,OpenRC/openrc,williamh/openrc,OpenRC/openrc,OpenRC/openrc,williamh/openrc