[ff][flux] Allow empty intersections in flux over surface

What this MR does / why does DuMux need it:

When solving free flow on the pore scale through void space (image data), it might happen that flux surfaces completely coincide with solid inclusions such that no intersections with the sub-grid can be found. In order to still allow to use this class it would be nice to have an option to treat such cases as no flow, meaning that we ignore empty intersections.