We were emitting drops when getMouseOverIndex was null, which could happen either when the drop is outside the container, or there were zero boxes to check for ordering. Treat these two cases separately, and continue to not emit drops when outside the container.

This reverts commit 97f8eb63, reversing
changes made to 2e017f06.

This is used because the sortable drop is also used for general dropping as well as sorting drop, so you need to be able to drop into a sortable list that does not contain any items.

Make the corresponding reorder code be more careful to check if there is a newIndex.