Make sure calling /context on an out-of-band membership event doesn't 500

[ara4n]

An out-of-band membership event (e.g. an invite or knock received over federation) is an outlier with no stream position or state group, so computing surrounding events or state for it fails with 'No state group for unknown or outlier event'.

Visibility already permits the user to see their own out-of-band membership events, and clients legitimately fetch them, e.g. to render a push notification for an invite after the server has already joined the user (auto-accepted knock). Return the event with an empty context instead of erroring.

This is helpful for clients trying to reason about knocks where they might call /context on a state event ID - for instance, Element X iOS's push extension (NSE) calls /context on every event ID it gets, so if you send a state event for autojoining a room you've knocked in, this would have 500'd.

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