When adding:
  -fx-fill: null

to the style of a Shape object, we get a black shape.
When invoking:
   Shape.setFill(null)
on the Shape object, we get a transparent (pixel and mouse) shape.

It would be good that -fx-fill: null behaves like Shape.setFill(null) because:
1) it would be consistent
2) it would allow creating a pixel and mouse transparent shape

In fact, #2 is the most important point ; today it is not possible to create
an outline (ie a shape whose content is pixel and mouse transparent) ;
this is a pretty annoying limitation.