Inventory: fix eval tests

lib/inventory/build-inventory/default.nix

@@ -189,7 +189,7 @@ let
           ) [ ] inventory.services;
         }
       ]
-    ) inventory.machines;
+    ) (inventory.machines or { });
 in
 {
   inherit buildInventory;

lib/inventory/tests/default.nix

@@ -88,9 +88,9 @@ in
         not_used_machine = builtins.length configs.not_used_machine;
       };
       expected = {
-        client_1_machine = 5;
-        client_2_machine = 5;
-        not_used_machine = 2;
+        client_1_machine = 6;
+        client_2_machine = 6;
+        not_used_machine = 3;
       };
     };