[youtube:channel] Fix the extraction of autogenerated channels

The ajax pages are empty, now it looks directly in the channel's /videos page

youtube_dl/extractor/youtube.py

@@ -1594,7 +1594,18 @@ class YoutubeChannelIE(InfoExtractor):
         # Download channel page
         channel_id = mobj.group(1)
         video_ids = []
+        url = 'https://www.youtube.com/channel/%s/videos' % channel_id
+        channel_page = self._download_webpage(url, channel_id)
+        if re.search(r'channel-header-autogenerated-label', channel_page) is not None:
+            autogenerated = True
+        else:
+            autogenerated = False
 
+        if autogenerated:
+            # The videos are contained in a single page
+            # the ajax pages can't be used, they are empty
+            video_ids = self.extract_videos_from_page(channel_page)
+        else:
             # Download all channel pages using the json-based channel_ajax query
             for pagenum in itertools.count(1):
                 url = self._MORE_PAGES_URL % (pagenum, channel_id)