Revert "openrc-init: ignore an empty string in argv[1]"

This reverts commit dec9ef200b.
This check is no longer needed since rc_runlevel_exists() is fixed.

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

src/rc/openrc-init.c

@@ -263,7 +263,7 @@ int main(int argc, char **argv)
 
 	printf("OpenRC init version %s starting\n", VERSION);
 
-	if (argc > 1 && argv[1][0] != '\0')
+	if (argc > 1)
 		default_runlevel = argv[1];
 	else
 		default_runlevel = NULL;