[dl:ytdl] fix exception when ytdl reports an error (#7301)

when using the HLS/DASH download functionality

gallery_dl/downloader/ytdl.py

@@ -200,6 +200,7 @@ class YoutubeDLDownloader(DownloaderBase):
             return None
 
         info_dict = {
+            "extractor": "",
             "id"       : video_id,
             "title"    : video_id,
             "formats"  : fmts,