[pbs] de-prioritize AD formats

Related: #2335

yt_dlp/extractor/pbs.py

@@ -545,7 +545,7 @@ class PBSIE(InfoExtractor):
                 for vid_id in video_id]
             return self.playlist_result(entries, display_id)
 
-        info = None
+        info = {}
         redirects = []
         redirect_urls = set()
 
@@ -660,6 +660,9 @@ class PBSIE(InfoExtractor):
                     'protocol': 'http',
                 })
                 formats.append(f)
+        for f in formats:
+            if (f.get('format_note') or '').endswith(' AD'):  # Audio description
+                f['language_preference'] = -10
         self._sort_formats(formats)
 
         rating_str = info.get('rating')