#3203 - Make rand of HParallelotope return a nonempty set

[schillic]

Closes #3203.

There is a small chance that the result is unbounded (this was also possible before).

julia> D = [1. 1; 1 1];
julia> offset = [1., 1, -1, -1];
julia> Q = HParallelotope(D, offset);
julia> P = HPolyhedron(vcat(Q.directions, -Q.directions), Q.offset);  # convert to polyhedron
julia> isbounded(P)
false

[mforets]

We could otherwise iterate until the set obtained is bounded.