nix flake show: Ignore empty attrsets

[roberth]

Motivation

For frameworks it's important that structures are as lazy as possible to prevent infinite recursions, performance issues and errors that aren't related to the thing to evaluate. filterAttrs makes the set of keys depend on the values. As a consequence, this function can't be used and the framework has to emit more attributes than desirable. However, these attributes with empty values are not useful to the user so we don't add them to the output.

Context

Closes #7285

Solves the need for these issues that I can't resolve without causing infinite recursions when people use self.

• Get rid of all the default empty outputs hercules-ci/flake-parts#66
• feat: don't output empty systems hercules-ci/flake-parts#113
• Filter away unset output attributes hercules-ci/flake-parts#59

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-2-14-0-released/25900/2

[oxalica]

Just bisected here. This breaks the output for nixpkgs. legacyPackages are no longer traversed even with --legacy.

$ nix flake show github:NixOS/nixpkgs/nixos-unstable --legacy
github:NixOS/nixpkgs/8f40f2f90b9c9032d1b824442cfbbe0dbabd0dbd
├───checks
│   └───x86_64-linux
│       └───tarball: derivation 'nixpkgs-tarball-23.05pre20230325.8f40f2f'
├───htmlDocs: unknown
├───legacyPackages
├───lib: unknown
└───nixosModules
    └───notDetected: NixOS module