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