[cassandra] TypeError: object of type 'int' has no len()

Sentry Issue: SWH-STORAGE-2X19

TypeError: object of type 'int' has no len()
(13 additional frame(s) were not displayed)
...
  File "swh/objstorage/factory.py", line 64, in get_objstorage
    return ObjStorage(**kwargs)
  File "swh/objstorage/factory.py", line 68, in _construct_filtered_objstorage
    return add_filters(get_objstorage(**storage_conf), filters_conf)
  File "swh/objstorage/factory.py", line 64, in get_objstorage
    return ObjStorage(**kwargs)
  File "swh/objstorage/backends/azure.py", line 389, in __init__
    prefix_lengths = set(len(prefix) for prefix in accounts)
  File "swh/objstorage/backends/azure.py", line 389, in <genexpr>
    prefix_lengths = set(len(prefix) for prefix in accounts)

Related to the following webapp request: https://webapp-cassandra.internal.softwareheritage.org/browse/origin/directory/?origin_url=deb://Debian/packages/linux&path=include/linux/soc/pxa&release=6.5.6-1

The query works for postgresql: https://archive.softwareheritage.org/browse/origin/directory/?origin_url=deb://Debian/packages/linux&path=include/linux/soc/pxa&release=6.5.6-1