Empty messages skip validation in protojson.decode_message

[julienr]

In protojson.decode_message, if encoded_message is an empty string, the validation will be skipped (and no ValidationError will be raised even if the message has some fields marked as required).

In https://github.com/google/protorpc/blob/master/protorpc/protojson.py#L201 , the following

if not encoded_message.strip():
  return message_type()

should be changed to

if not encoded_message.strip():
  message = message_type()
  message.check_initialized()
  return message