fix: 工作流图片不出来问题

src/pages/bsmanager/workOrder/workflowEdit/WorkflowHomePage.jsx

@@ -120,7 +120,7 @@ const WorkflowHomePage = () => {
         let list = res.data.map((item, index) => {
           item.children.forEach(ele => {
             flowNameList.push(ele.FlowName);
-            ele.PreviewImage = getImgUrl(ele.PreviewImage);
+            ele.PreviewImage = ele.PreviewImage ? getImgUrl(ele.PreviewImage) : '';
           });
           item.isOld = true;
           item.bgType = (index + 1) % 5;