[bilibili] Fix extraction for videos without backup_url (#10647)

ChangeLog

@@ -1,6 +1,7 @@
 version <unreleased>
 
 Extractors
+* [bilibili] Fix extraction for specific videos (#10647)
 + [nbc] Add support for NBC Olympics (#10361)
 
 

youtube_dl/extractor/bilibili.py

@@ -67,7 +67,7 @@ class BiliBiliIE(InfoExtractor):
                 'url': durl['url'],
                 'filesize': int_or_none(durl['size']),
             }]
-            for backup_url in durl['backup_url']:
+            for backup_url in durl.get('backup_url', []):
                 formats.append({
                     'url': backup_url,
                     # backup URLs have lower priorities