Bugfixes in derived collections (rxui4)

This is a backport of #337 for ReactiveUI 4.x

> This fixes two issues with the recent move support (#327) in derived collections. The first is a typo or a copy-paste-fail on my part. When newPositionForItem receives a list with a single item in it there's nowhere that the item can  move so it should return index 0 but instead returned 1.
> 
> The second part is that derived collections with a filter but no orderer may publish a move notification with the same source and destination index (ie a noop move).