nextboot(8): Fix behavior on non-ZFS /boot systems

Fix unquoted test for an empty value, which broke nextboot(8) on non-ZFS /boot
systems after r365938.

Discussed with:	allanjude, tsoome
X-MFC-With:	r365938

sbin/reboot/nextboot.sh

@@ -116,7 +116,7 @@ set -e
 
 nextboot_tmp=$(mktemp $(dirname ${nextboot_file})/nextboot.XXXXXX)
 
-if [ -n ${zfs} ]; then
+if [ -n "${zfs}" ]; then
 	zfsbootcfg -z ${zfs} -n freebsd:nvstore -k nextboot_enable -v YES
 	cat >> ${nextboot_tmp} << EOF
 $kenv