Fix a null deref when visual debugging is showing a sub-tree node with no children

Author: achynes

Assets/com.fluid.behavior-tree/Editor/BehaviorTree/Printer/VisualTask.cs

@@ -25,7 +25,7 @@ public VisualTask (ITask task, IGraphContainer parentContainer) {
 
             AddBox(container);
 
-            if (task.Children != null) {
+            if (task.Children != null && task.Children.Count > 0) {
                 var childContainer = new GraphContainerHorizontal();
                 foreach (var child in task.Children) {
                     _children.Add(new VisualTask(child, childContainer));