handle unconnected graphs in Stress

[hexaeder]

using CairoMakie,GraphMakie,NetworkLayout
sgkeys = [:bull, :chvatal, :cubical, :desargues,
            :diamond, :dodecahedral, :frucht, :heawood,
            :house, :housex, :icosahedral, :karate, :krackhardtkite,
            :moebiuskantor, :octahedral, :pappus, :petersen,
            :sedgewickmaze, :tetrahedral, :truncatedcube,
            :truncatedtetrahedron, :truncatedtetrahedron_dir, :tutte]
gs = filter(!is_directed, smallgraph.(sgkeys))
absdim = mapreduce(nv, +, gs)
adj = zeros(Int, absdim, absdim);
let i = 1
    for g in gs
        r = i:i+nv(g)-1
        adj[r, r] .= adjacency_matrix(g)
        i += nv(g)
    end
end
g = SimpleGraph(adj)
graphplot(g; layout=Stress())