FIX: Don't error out on deleted users

[Drenmi]

What is the problem?

The GuardianExtension#poster_group_allow_translate? method restricts and determines whether a post can be translated based on the poster's group membership. There are reports that this errors out when deleting spammers.

The problem is that when a spammer is deleted, their post user_id is nullified, and because the above method doesn't account for this case, we try to call #in_any_groups? on nil, which errors out.

This PR fixes that by explicitly accounting for the case of a post with a nil user ID.

---

We have to make a decision here, since we're losing the history of which groups the poster was in, whether the post should be translatable or not. I'm defaulting to false here. It should be a minor thing as deleting a spam user should hide or delete their post as well.