graph_blockmodel.hh: Fix bug with neighbour sampler

This fixes a bug with isolated nodes that contain only self-loops.

src/graph/generation/sampler.hh

@@ -93,6 +93,7 @@ public:
     }
 
     size_t size() const { return _items.size(); }
+    bool empty() const { return _items.empty(); }
 
 private:
 

src/graph/inference/graph_blockmodel.hh

@@ -1261,9 +1261,10 @@ public:
         // attempt random block
         size_t s = uniform_sample(_candidate_blocks, rng);
 
-        if (!std::isinf(c) && total_degreeS()(v, _g) > 0)
+        auto& sampler = _neighbour_sampler[v];
+        if (!std::isinf(c) && !sampler.empty())
         {
-            auto u = sample_neighbour(_neighbour_sampler[v], rng);
+            auto u = sample_neighbour(sampler, rng);
             size_t t = _b[u];
             double p_rand = 0;
             if (c > 0)