fix(接口测试): 修复断言添加子字段无效的缺陷

--bug=1019419 --user=王孝刚
【接口测试】github#19488，接口自动化TEST界面断言文档结构时“添加子字段”按钮功能无效
https://www.tapd.cn/55049933/s/1289916

api-test/frontend/src/business/definition/components/assertion/document/DocumentBody.vue

@@ -481,7 +481,7 @@ export default {
       let newRow = this.getNewRow();
       newRow.idList = [row.id, newRow.id];
       newRow.parentId = row.id;
-      if (this.mapData.has(row.id) && this.mapData.get(row.id).length > 0) {
+      if (this.mapData.has(row.id)) {
         this.mapData.get(row.id).push(newRow);
       } else {
         this.getChild(this.originalData, row);