etc/systemd/zfs-mount-generator: don't fail if no cached pools

If $FSLIST exists but is empty, the generator fails with
  sort: cannot read: '/etc/zfs/zfs-list.cache/*':
  No such file or directory

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11915

etc/systemd/system-generators/zfs-mount-generator.in

@@ -28,6 +28,7 @@ set -e
 FSLIST="@sysconfdir@/zfs/zfs-list.cache"
 
 [ -d "${FSLIST}" ] || exit 0
+[ "$(echo "${FSLIST}"/*)" = "${FSLIST}/*" ] && exit 0
 
 do_fail() {
   printf 'zfs-mount-generator: %s\n' "$*" > /dev/kmsg