message: Early return for nest flatten when the input is None
diff: @@ -335,6 +335,8 @@ def flatten(structure, expand_composites=False):
  Raises:
  TypeError: The nest is or contains a dict with non-sortable keys.
  """
  + if structure is None:
  + return [None]
  return _pywrap_utils.Flatten(structure, expand_composites)
  