 can not use "not res" to judge if the 'res' is empty table


 the Synopsis in readme  use
`local res, err, errcode, sqlstate =
                    db:query("drop table if exists cats")
                if not res then
                    ngx.say("bad result: ", err, ": ", errcode, ": ", sqlstate, ".")
                    return
                end`
"not res" cannot judge 
you shoud use next(use) or calculate the length to judge


