修复部分情况下获取消息数据抛出异常的问题

Author: ChengZhiMeow

mhdf-bot/src/main/java/cn/chengzhiya/mhdfbot/bot/onebot/MHDFOneBot.java

@@ -145,6 +145,7 @@ public AbstractMessageEvent getMsg(Long messageId) {
 
         JSONObject data = Objects.requireNonNull(JSONObject.parseObject(this.httpClient.post("get_msg", body.toString())))
                 .getJSONObject("data");
+        if (data == null) return null;
 
         return switch (data.getString("message_type")) {
             case "group" -> new GroupMessageEvent(data);