add empty set of hydra tests

outputs.nix

@@ -76,5 +76,5 @@
   ) // (nixpkgs.lib.mapAttrs'
     (name: config: nixpkgs.lib.nameValuePair name config.activation-script)
     self.hmConfigurations
-  );
+  ) // (let tests = import ./tests; in flake-utils.lib.eachDefaultSystem tests);
 }

tests/default.nix

@@ -0,0 +1,3 @@
+system:
+{
+}