beego: fix log output when SetLogger has error

logs/conn.go

@@ -29,6 +29,9 @@ func NewConn() LoggerInterface {
 // init connection writer with json config.
 // json config only need key "level".
 func (c *ConnWriter) Init(jsonconfig string) error {
+	if len(jsonconfig) == 0 {
+		return nil
+	}
 	err := json.Unmarshal([]byte(jsonconfig), c)
 	if err != nil {
 		return err