`lib.all (_: false) [ ]` is `true`, which is not the semantics we want
here, and interacts poorly with `boot.zfs.extraPools` where
`getPoolFilesystems` may return `[]`. So explicitly handle this case.
This does mean there isn’t a straightforward way to disable auto-import
with `extraPools` and no associated `fileSystems` while keeping the unit
around, but that’s probably okay for now.
See https://github.com/NixOS/nixpkgs/issues/364995.
Broken in 7f8278a264.
4ed27101c6