Could you explain what exactly you mean by a stateless RNN and what network topology you are using? Is rnn in your case a cell or a complete RNN? Do you pass a whole sequence or only one timestep input?
If the initial hidden state is not passed (None) internally a zero vector is used as the first hidden state. If conditioning on the initial hidden state is not beneficial it is possible that the ‘performace’ of the model is better than using an additional context vector.