message: add check for 0 funcs and raise error
diff: @@ -516,12 +516,17 @@ class TFLiteConverterV2(TFLiteConverterBase):
  Raises:
  ValueError:
  + No concrete functions is sepecfied.
  Multiple concrete functions are specified.
  Input shape is not specified.
  Invalid quantization parameters.
  """
  # TODO(b/130297984): Add support for converting multiple function.
  - if len(self._funcs) != 1:
  +
  + if len(self._funcs) == 0:
  + raise ValueError("No ConcreteFunction is sepecfied.")
  +
  + if len(self._funcs) > 1:
  raise ValueError("This converter can only convert a single "
  "ConcreteFunction. Converting multiple functions is "
  "under development.")
  