Allow SubGraphOutputNode to supply shader data to master preview view

com.unity.shadergraph/Editor/Drawing/PreviewManager.cs

 
             if (masterRenderData.shaderData == null && masterNode != null)
                 masterRenderData.shaderData = shaderData;
+
+            var subGraphOutputNode = node as SubGraphOutputNode;
+
+            if (masterRenderData.shaderData == null && subGraphOutputNode != null)
+                masterRenderData.shaderData = shaderData;
         }
 
         void OnNodeModified(INode node, ModificationScope scope)