nixos/self-deploy: make nixAttribute nullable (#125617)

Allows using a nix file that directly provides the derivation

nixos/modules/services/system/self-deploy.nix

@@ -37,7 +37,9 @@ in
     };
 
     nixAttribute = lib.mkOption {
-      type = lib.types.str;
+      type = with lib.types; nullOr str;
+
+      default = null;
 
       description = ''
         Attribute of `nixFile` that builds the current system.