fix(测试跟踪): 用例编辑模块显示有误

--bug=1023686 --user=陈建星 【测试跟踪】功能用例-创建用例-选择模块-保存后模块显示为暂无 https://www.tapd.cn/55049933/s/1341112

test-track/frontend/src/business/case/components/case/CaseBaseInfo.vue

@@ -573,7 +573,10 @@ export default {
           return node;
         } else {
           if (node.children && node.children.length > 0) {
-            return this.findTreeNode(node.children);
+            let findNode = this.findTreeNode(node.children);
+            if (findNode != null) {
+              return findNode;
+            }
           }
         }
       }