彻底解决mp4_as_player设置为1时，on_stream_none_reader回调无效流信息的问题 (#4725)

src/Common/MediaSource.h

@@ -573,5 +573,11 @@ private:
     toolkit::ObjectStatistic<MediaSource> _statistic;
 };
 
+struct MediaSourceNull : public MediaSource {
+    MediaSourceNull(const MediaTuple &tuple = { "vhost", "app", "stream", "" })
+        : MediaSource("schema", tuple) { };
+    int readerCount() override { return 0; }
+};
+
 } /* namespace mediakit */
 #endif //ZLMEDIAKIT_MEDIASOURCE_H