vertices_list for AbstractHPolygon with infeasible constraints is incorrect

[schillic]

vertices_list does not work correctly if the constraints are infeasible.
The result should be an empty list, but it is not.

julia> P = HPolygon([
    HalfSpace([1.0, 1.0], 0.0),
    HalfSpace([-1.0, 0.0], -1.0),
    HalfSpace([0.0, -1.0], -1.0)]);

julia> vertices_list(P)
3-element Array{Array{Float64,1},1}:
 [1.0, -1.0]
 [1.0, 1.0] 
 [-1.0, 1.0]

The reason is that we take the intersection of lines without caring for the direction of the normal vector.
This has serious consequences because the emptiness check relies on it.