Various fixes to support posts with no user (PR #8877)

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Do you know when this could be happening? When are we deleting an user but forgetting to delete topic_links records?

As part of the post processing pipeline we are calling TopicLink.extract_from. When processing a post with no user, that method will raise an exception as it attempts to create TopicLink records (because user_id is nil).

Maybe we should instead bypass the post processing altogether if there are no user associated to a post?

I do not think it makes sense skipping post processing because some things may not work anymore, but I made it skip TopicLink creation which was the problem in the first place.