Skip processing Update activities on unknown accounts (#15514)
Skip processing Update activities on unknown accounts (#15514)

This also skips fetching the actor completely.

This will be useful if we end up distributing Update activities linked to
account suspensions more widely (they are currently only delivered to
the suspended account's followers), as currently, instances not knowing
about the suspended account would fetch it to then process the suspension.

Co-authored-by: Claire <82b964c50e9d1d222e48cf5046e6484a966a7b07@sitedethib.com>
koba-lab/mastodon,koba-lab/mastodon,koba-lab/mastodon,koba-lab/mastodon