fix null description

mooctest-site-server/src/main/java/cn/iselab/mooctest/site/web/logic/impl/GeneralCalculateScoreComponent.java

@@ -149,7 +149,8 @@ public class GeneralCalculateScoreComponent {
     for(Node node:basicNode.getNodes()){
       if(node.getName().equals(nodeName)){
         String description = node.getDescription();
-        if((!description.isEmpty())&&(description.contains("\"weight\":"))){//描述中含有权重字段，暂时这样存放单个node的权重
+        if((!(description==null))&&(description.contains("\"weight\":")))
+        {//描述中含有权重字段，暂时这样存放单个node的权重
           NodeExtends nodeExtension = JSONUtil.json2Obj(description, NodeExtends.class);
           singleNodeWeight = nodeExtension.getWeight();
         }