markchatasread on empty conversation returns an error

[jjeffryes]

When http://localhost:4002/ob/markchatasread?subject=QmZw2nQa9KKR8LnVMoKdJL7nPDGd7PA85hYMrpZuqS4TYd is called on a conversation with no messages, it returns a 500 status with the following response:

{
    "success": false,
    "reason": "sql: Scan error on column index 0: converting driver.Value type \u003cnil\u003e ('\u003cnil\u003e') to a int: invalid syntax"
}

It should return a 200 status with an empty response.