Select top/bottom layer with Cmd-Up/Down when no layers are selected

When a layer is already selected, it's easy to navigate the layer stack with Cmd-Up or Cmd-Down (or Ctrl-Up/Down on Windows).

However, when no layers are selected, those keyboard shortcuts have no effect, and the user has to switch over to the mouse to select one or more layers before being able to return to navigating via shortcuts.

It would he helpful if, when no layers are selected, Cmd-Up selected the top-most layer and Cmd-Down selected the bottom-most layer.