Fix: allow empty choices

Some Azure GPT Models seem to return unexpected empty choices array, when streaming, then proceed normaly

src/ChatGPTClient.js

@@ -359,7 +359,7 @@ ${botMessage.message}
  if (progressMessage === '[DONE]') {
  return;
  }
- const token = this.isChatGptModel ? progressMessage.choices[0].delta.content : progressMessage.choices[0].text;
+ const token = this.isChatGptModel ? progressMessage.choices[0]?.delta.content : progressMessage.choices[0]?.text;
  // first event's delta content is always undefined
  if (!token) {
  return;