make repl_string work on any dict like object.

cylc/flow/parsec/config.py

@@ -212,7 +212,7 @@ def jdump(
                 keys = []
             cfg = self.get(keys, sparse)
             if none_str:
-                cfg.repl_val(cfg, None, none_str)
+                OrderedDictWithDefaults.repl_val(cfg, None, none_str)
             data = json.dumps(cfg, indent=indent)
 
         print(data, file=handle or sys.stdout)