blockmodel: Fix issue with sampling neighbours for degree-zero nodes

src/graph/inference/graph_blockmodel.hh

@@ -1298,9 +1298,10 @@ public:
 
     size_t random_neighbour(size_t v, rng_t& rng)
     {
-        if (_neighbour_sampler[v].size() == 0)
+        auto& sampler = _neighbour_sampler[v];
+        if (sampler.empty())
             return v;
-        return sample_neighbour(_neighbour_sampler[v], rng);
+        return sample_neighbour(sampler, rng);
     }
 
     // Computes the move proposal probability

src/graph/inference/graph_blockmodel_util.hh

@@ -1528,12 +1528,6 @@ void build_neighbour_sampler(Vertex v, SMap& sampler, Eprop& eweight, Graph& g,
                                       // 2 * eweight[e]
     }
 
-    if (probs.empty())
-    {
-        neighbours.push_back(v);
-        probs.push_back(1.);
-    }
-
     sampler = Sampler<Vertex, mpl::false_>(neighbours, probs);
 };