Handle empty reply text

[MrOrz]

Inputting this will break staging chatbot:

📃 查看這篇的回應:
https://dev.cofacts.org/article/AV1vp0l4yCdS-nWhuc4e

The error in console says:

{
  "message": "A message (messages[2]) in the request body is invalid",
  "details": [
    {
      "message": "must be non-empty text",
      "property": "/contents/5/body/contents/0/text"
    },
    {
      "message": "must be non-empty text",
      "property": "/contents/6/body/contents/0/text"
    },
    {
      "message": "must be non-empty text",
      "property": "/contents/7/body/contents/0/text"
    }
  ]
}

This is because there are some replies with empty text (probably caused by a previous bug.)
The reply sending mechanism should prevent empty text from being sent as an option.