fix problems with null json node. (#1785)

src/treelearner/serial_tree_learner.cpp

@@ -688,12 +688,16 @@ int32_t SerialTreeLearner::ForceSplits(Tree* tree, Json& forced_split_json, int*
     right = Json();
     if ((pair.first).object_items().count("left") > 0) {
       left = (pair.first)["left"];
+      if (left.object_items().count("feature") > 0 && left.object_items().count("threshold") > 0) {
         q.push(std::make_pair(left, *left_leaf));
       }
+    }
     if ((pair.first).object_items().count("right") > 0) {
       right = (pair.first)["right"];
+      if (right.object_items().count("feature") > 0 && right.object_items().count("threshold") > 0) {
         q.push(std::make_pair(right, *right_leaf));
       }
+    }
     result_count++;
     *(cur_depth) = std::max(*(cur_depth), tree->leaf_depth(*left_leaf));
   }