the stripe api returns a response empty

Where exactly are you getting that example from? It looks incomplete.

You should just save the output of the API call to some variable, then return the client_secret for example:

$intent = $stripe->paymentIntents->create([
  'amount' => 1200,
  'currency' => 'usd',
  'payment_method_types' => ['card']
]);

$this->response([ 'success' =>'1' ,'status' => 'success', 'message' => $intent->client_secret], REST_Controller::HTTP_OK);