nix flake show: Ignore empty attrsets

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. As a consequence, they have
to emit more attributes than strictly (sic) necessary.
However, these attributes with empty values are not useful to the user
so we omit them.

tests/flakes/show.sh

@@ -37,3 +37,30 @@ in
 assert show_output.legacyPackages.${builtins.currentSystem}.hello.name == "simple";
 true
 '
+
+# Test that attributes are only reported when they have actual content
+cat >flake.nix <<EOF
+{
+  description = "Bla bla";
+
+  outputs = inputs: rec {
+    apps.$system = { };
+    checks.$system = { };
+    devShells.$system = { };
+    legacyPackages.$system = { };
+    packages.$system = { };
+    packages.someOtherSystem = { };
+
+    formatter = { };
+    nixosConfigurations = { };
+    nixosModules = { };
+  };
+}
+EOF
+nix flake show --json --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output == { };
+true
+'