Fix the test for GoPro extractor.

gopro.com returns empty string as author/track for some videos; consider
such attributes to be missing.

Signed-off-by: Alexey Neyman <stilor@att.net>

yt_dlp/extractor/gopro.py

@@ -3,12 +3,20 @@
     int_or_none,
     remove_end,
     str_or_none,
+    strip_or_none,
     try_get,
     unified_timestamp,
     url_or_none,
 )
 
 
+# gopro.com returns empty string as author/track for some video. Treat it as
+# if it were null.
+def nonempty_or_none(s):
+    v = strip_or_none(s)
+    return None if v == '' else v
+
+
 class GoProIE(InfoExtractor):
     _VALID_URL = r'https?://(www\.)?gopro\.com/v/(?P<id>[A-Za-z0-9]+)'
 
@@ -98,8 +106,8 @@ def _real_extract(self, url):
                 try_get(metadata, lambda x: x['account']['nickname'])),
             'duration': int_or_none(
                 video_info.get('source_duration')),
-            'artist': str_or_none(
+            'artist': nonempty_or_none(
                 video_info.get('music_track_artist')),
-            'track': str_or_none(
+            'track': nonempty_or_none(
                 video_info.get('music_track_name')),
         }