safe guard empty message will not produce "BUG" message, it's legit

wechatMsg.go

 }
 
 func (m *InWechatMsg) replyText(s string) {
+	if s == "" && m.replied == true {
+		return //skip safeguard empty message
+	}
 	str, _ := BuildTextMsg(m.header.FromUserName, s)
 	m.replyXML(str)
 }