fix(测试计划): 关联无责任人用例时报错#1005374

--bug=1005374 --user=lyh【github#4960】导入的用例无责任人时无法在计划中进行关联
https://www.tapd.cn/55049933/s/1028453

backend/src/main/java/io/metersphere/track/service/TestPlanService.java

@@ -461,7 +461,8 @@ public class TestPlanService {
         TestCaseExample testCaseExample = new TestCaseExample();
         testCaseExample.createCriteria().andIdIn(testCaseIds);
         List<TestCase> testCaseList = testCaseMapper.selectByExample(testCaseExample);
-        Map<String, String> userMap = testCaseList.stream().collect(Collectors.toMap(TestCase::getId, TestCase::getMaintainer));
+        Map<String, String> userMap = testCaseList.stream()
+                .collect(HashMap::new, (m,v)-> m.put(v.getId(), v.getMaintainer()), HashMap::putAll);
 
         SqlSession sqlSession = sqlSessionFactory.openSession(ExecutorType.BATCH);
         TestPlanTestCaseMapper batchMapper = sqlSession.getMapper(TestPlanTestCaseMapper.class);