refactor(测试跟踪): xmind用例导入，没有合法的用例提示优化

--bug=1039371 --user=陈建星 【测试跟踪】github#29824，测试跟踪用例导入xmind模版，连续两次点击导入按钮，会先提示导入失败，再次提示导入成功，实际文件并为导入成功 https://www.tapd.cn/55049933/s/1517834

test-track/backend/src/main/java/io/metersphere/service/TestCaseService.java

@@ -1215,8 +1215,10 @@ public class TestCaseService {
                     }
                     httpRequest.setAttribute("ms-req-title", String.join(",", names));
                     httpRequest.setAttribute("ms-req-source-id", JSON.toJSONString(ids));
+                } else {
+                    MSException.throwException(Translator.get("no_legitimate_case_tip"));
+                    return null;
                 }
-
             }
             xmindParser.clear();
         } catch (Exception e) {