Restore the ability to handle None as a key. We now use _null_key instead of None to indicate the end-of-refs. This means we now check that _null_key isn't used as an actual key. This slows us down from 7.1 => 7.3s or so. Interestingly, the globals lookup of _null_key was faster than node is self._lru (7.5s+). I was a bit surprised at that.