fix(测试跟踪): 测试计划关联场景用例报错

--bug=1027977 --user=陈建星 【测试跟踪】新建测试计划-关联接口场景-选择所有数据-场景未添加到测试计划内 https://www.tapd.cn/55049933/s/1396413

api-test/backend/src/main/java/io/metersphere/service/plan/TestPlanScenarioCaseService.java

@@ -248,6 +248,7 @@ public class TestPlanScenarioCaseService {
             testPlanApiScenario.setTestPlanId(request.getPlanId());
             testPlanApiScenario.setCreateTime(System.currentTimeMillis());
             testPlanApiScenario.setUpdateTime(System.currentTimeMillis());
+            newEnvMap.remove(null);
             String environmentJson = JSON.toJSONString(newEnvMap);
             if (StringUtils.equals(envType, EnvironmentType.JSON.name())) {
                 testPlanApiScenario.setEnvironment(environmentJson);