Superflous/confusing line

[BrianHenryIE]

I spent far too much time looking at this line before realising it doesn't do anything!

$user_id is already false here.

[bradyvercher]

I can't recall if that was added for a specific reason or if it was just to be explicit what the return value should be (without returning early), but some callbacks do pass unexpected data into that filter. I think null is the only one that would ever make it that far and it wouldn't be handled any different once it bubbles back up to WordPress, so it's probably safe to remove.