code: def normalize_missing(xs):
      
      if isinstance(xs, dict):
          for k, v in xs.items():
              xs[k] = normalize_missing(v)
      elif isinstance(xs, (list, tuple)):
          xs = [normalize_missing(x) for x in xs]
      elif isinstance(xs, six.string_types):
          if xs.lower() in ["none", "null"]:
              xs = None
          elif xs.lower() == "true":
              xs = True
          elif xs.lower() == "false":
              xs = False
      return xs
docs: Normalize missing values to avoid string 'None' inputs.