Add a trick to handle null sql query results

[liyuankui]

[xaprb]

This is interesting, but I don't know how to think about benefits and drawbacks, or other difference. For example, what happens if there is an error? I think we need a little more narrative to introduce this technique and explain why it wouldn't be an officially recommended technique. If this works fine, why did the database/sql authors create the built-in nullable types?