fix: add nil check in deer-go LoggerCallback to prevent panic (#89)

flow/agent/deer-go/biz/infra/logger.go

@@ -125,10 +125,12 @@ func (cb *LoggerCallback) pushMsg(ctx context.Context, msgID string, msg *schema
 
 func (cb *LoggerCallback) OnStart(ctx context.Context, info *callbacks.RunInfo, input callbacks.CallbackInput) context.Context {
 	if inputStr, ok := input.(string); ok {
+		if cb.Out != nil {
 			cb.Out <- "\n==================\n"
 			cb.Out <- fmt.Sprintf(" [OnStart] %s ", inputStr)
 			cb.Out <- "\n==================\n"
 		}
+	}
 	return ctx
 }