achievements_unlocked_already does not differentate if a user has no access to achievemnet or has not unlocked an achievement

I think this approach makes sense. We should return something in either case.

I wonder if we'll need to change this bit further down in the function:

if (isset($achievement_id)) { // return data about a specific unlock if requested.
    return isset($unlocks[$uid][$achievement_id]) ? $unlocks[$uid][$achievement_id] : NULL;
  }
  else { // all of 'em.
    return $unlocks[$uid];
  }

Since the return values here are mixed. Instead of NULL perhaps an empty array, or keeping things consistent from access control above and make it a FALSE instead. Thoughts?