nixos/clevis: guard zfs code behind config.clevis.boot.initrd.enable & skip filesystem with device = null

[JulienMalka]

Description of changes

The rationale behind the expression before was that clevisDatasets is empty if clevis is disabled. The problem is that the evaluation of clevisDatasets can fail when the users has filesystems with device = null (if you are mounting by label for example), currently causing issues for users that are not using clevis.

This introduces two changes:

1. Isolates any user without clevis enabled from clevis implementation. The only offending case was in zfs.nix.
2. Skips filesystems with device = null. Better support for clevis devices by label will be implemented in a followup.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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