Fix backfilled events being rejected for no `state_groups` (#10439)

Reproducible on a federated homeserver when there is a membership auth event as a floating outlier. Then when we try to backfill one of that persons messages, it has missing membership auth to fetch which caused us to mistakenly replace the `context` for the message with that of the floating membership `outlier` event. Since `outliers` have no `state` or `state_group`, the error bubbles up when we continue down the persisting route: `sqlite3.IntegrityError: NOT NULL constraint failed: event_to_state_groups.state_group`

Call stack:

```
backfill
_auth_and_persist_event
_check_event_auth
_update_auth_events_and_context_for_auth
```