message: Handle no default arguments in ragged ops
diff: @@ -539,6 +539,7 @@ def _ragged_op_signature(op, ragged_args):
  arg_names[pos] = '**' + arg_names[pos] + '**'
  # Add argument defaults.
  + if argspec.defaults is not None:
  for pos in range(-1, -len(argspec.defaults) - 1, -1):
  arg_names[pos] += '=`{!r}`'.format(argspec.defaults[pos])
  