Inventory/interface: optional instances, allow to use modules without inventory

clanModules/borgbackup/roles/client.nix

@@ -5,7 +5,8 @@
   ...
 }:
 let
-  instances = config.clan.inventory.services.borgbackup;
+  # Instances might be empty, if the module is not used via the inventory
+  instances = config.clan.inventory.services.borgbackup or { };
   # roles = { ${role_name} :: { machines :: [string] } }
   allServers = lib.foldlAttrs (
     acc: _instanceName: instanceConfig:

clanModules/borgbackup/roles/server.nix

@@ -4,9 +4,11 @@ let
   machineDir = clanDir + "/machines/";
   inherit (config.clan.core) machineName;
 
-  instances = config.clan.inventory.services.borgbackup;
-
-  # roles = { ${role_name} :: { machines :: [string] } }
+  # Instances might be empty, if the module is not used via the inventory
+  #
+  # Type: { ${instanceName} :: { roles :: Roles } }
+  #   Roles :: { ${role_name} :: { machines :: [string] } }
+  instances = config.clan.inventory.services.borgbackup or { };
 
   allClients = lib.foldlAttrs (
     acc: _instanceName: instanceConfig:

nixosModules/clanCore/inventory/interface.nix

@@ -48,6 +48,7 @@ let
 in
 {
   options.clan.inventory.services = lib.mkOption {
+    default = { };
     description = ''
       Configuration for each inventory service.