[utils] unified_strdate: Return None if the date format can't be recognized (fixes #7340)

This issue was introduced with ae12bc3ebb, it returned 'None'.

youtube_dl/utils.py

@@ -911,7 +911,8 @@ def unified_strdate(date_str, day_first=True):
         timetuple = email.utils.parsedate_tz(date_str)
         if timetuple:
             upload_date = datetime.datetime(*timetuple[:6]).strftime('%Y%m%d')
-    return compat_str(upload_date)
+    if upload_date is not None:
+        return compat_str(upload_date)
 
 
 def determine_ext(url, default_ext='unknown_video'):