Check the status of one ask by ask_id. Until someone has actually gone and answered, answer is null — do not invent an answer in its place. Tell the user it is still waiting for a person. The entire point of this service is that nothing is made up. 按 ask_id 查一单到哪一步了。没人答之前 answer 是 null——这时候不要替他编一个答案，如实告诉用户还在等人。