move checks if targetHost/buildHost is set to cli

nixosModules/clanCore/networking.nix

@@ -14,7 +14,8 @@
           - user@machine2.example.com
           - root@example.com:2222&IdentityFile=/path/to/private/key
       '';
-      type = lib.types.str;
+      default = null;
+      type = lib.types.nullOr lib.types.str;
     };
     buildHost = lib.mkOption {
       description = ''

nixosModules/clanCore/outputs.nix

@@ -20,13 +20,13 @@
           '';
         };
         deployment.buildHost = lib.mkOption {
-          type = lib.types.str;
+          type = lib.types.nullOr lib.types.str;
           description = ''
             the hostname of the build host where nixos-rebuild is run
           '';
         };
         deployment.targetHost = lib.mkOption {
-          type = lib.types.str;
+          type = lib.types.nullOr lib.types.str;
           description = ''
             the hostname of the target host to be deployed to
           '';