Merge branch '61264-dast-report-error' into 'master'

Backporting EE fix

Closes #61264

See merge request gitlab-org/gitlab-ce!29004

spec/fixtures/security-reports/master/gl-dast-report.json

 {
-  "site": {
+  "site": [
+    {
       "alerts": [
         {
           "sourceid": "3",
@@ -34,7 +35,8 @@
       "@port": "80",
       "@host": "bikebilly-spring-auto-devops-review-feature-br-3y2gpb.35.192.176.43.xip.io",
       "@name": "http://bikebilly-spring-auto-devops-review-feature-br-3y2gpb.35.192.176.43.xip.io"
-  },
+    }
+  ],
   "@generated": "Fri, 13 Apr 2018 09:22:01",
   "@version": "2.7.0"
 }