"assert self.tags is not None" fails

https://sentry.softwareheritage.org/organizations/swh/issues/8955/?referrer=phabricator_plugin

AssertionError: 
  File "swh/loader/core/loader.py", line 339, in load
    self.store_data()
  File "swh/loader/bzr/loader.py", line 326, in store_data
    assert self.tags is not None

Loading failure, updating to `failed` status

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Migrated from T3963 (view on Phabricator)