fix(dracut-initramfs-restore.sh): check if SELINUXTYPE is set

fix(dracut-initramfs-restore.sh): check if SELINUXTYPE is set

It can happen that SELINUXTYPE is not set, e.g., in Fedora/RHEL
installation images /etc/selinux/config is an empty file. That means
that setfiles fails, which makes dracut-initramfs-restore fail too
because it uses "set -x", which means that dracut-shutdown.service
fails. But, after commit 7ab1d00227cad6f1b86ba01fdc766769faebb031, a
failure of dracut-shutdown.service means that the final switch back into
initrd is skipped. Let's just add an additional sanity check to
workaround that.