Currently if no layer is selected, creating new layer above/below current would not work. Thats is understandable.

Suggestion: (if no layer is selected)

Creating new layer above current - create new layer above the highest layer

Creating new layer below current - create new layer below bove the lowest layer

I think adding a separate third option that says "Create new layer at the Top" that just creates a new layer above the highest layer might be better than having one button act in different way in different situations. This will be a much welcome addition.