vertices_list of an empty HParallelotope is non-empty

[schillic]

julia> H = [1.0 1.0; 0.0 1.0];
julia> o = [2.5, 2.0, -4.0, -1.0];
julia> Pl = HParallelotope(H, o);  # this set is empty

julia> vertices_list(Pl)
4-element Vector{Vector{Float64}}:
 [2.0, 2.0]
 [0.5, 2.0]
 [1.5, 1.0]
 [3.0, 1.0]

~The question is whether we want to fix this because in a sense the set is illegal: we do not allow empty parallelotopes conceptually.~ We want to fix vertices_list.