[Add] BotCommandProcess 新增'色图'功能以随机从排行榜中获取一张图片;

[Add] BufferMessageEvent 增加用于获取Sender发送内容的Event实现;
[Change] BotEventHandler Handler现在允许命令处理方法不返回消息(返回'null');

src/main/java/net/lamgc/cgj/bot/event/BotEventHandler.java

@@ -224,7 +224,7 @@ public class BotEventHandler implements EventHandler {
             }
         }
         long processTime = System.currentTimeMillis() - time;
-        if(Objects.requireNonNull(result) instanceof String && !isMute(event.getFromGroup())) {
+        if(!Objects.isNull(result) && result instanceof String && !isMute(event.getFromGroup())) {
             try {
                 event.sendMessage((String) result);
             } catch (Exception e) {