fix(性能测试): 修复删除测试之后再删除报告报错的问题

backend/src/main/java/io/metersphere/performance/service/PerformanceReportService.java

@@ -96,6 +96,7 @@ public class PerformanceReportService {
 
         LogUtil.info("Delete report started, report ID: %s" + reportId);
 
+        if (loadTest != null) {
             try {
                 final Engine engine = EngineFactory.createEngine(loadTest);
                 if (engine == null) {
@@ -115,6 +116,7 @@ public class PerformanceReportService {
                 loadTest.setStatus(PerformanceTestStatus.Saved.name());
                 loadTestMapper.updateByPrimaryKeySelective(loadTest);
             }
+        }
 
         // delete load_test_report_result
         LoadTestReportResultExample loadTestReportResultExample = new LoadTestReportResultExample();