Add compatibility for SCMTriggerItem-related changes in 1.568

Untested, unfortunately. Should affect Maven Projects (or generally anything with `SCM` based on core <1.568 and not a `Project`) though.

Issue report:
https://issues.jenkins-ci.org/browse/JENKINS-5413?focusedCommentId=205518&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-205518

Cause: https://github.com/jenkinsci/jenkins/commit/98ee11a28903e3d3003d912f143150bcf1b37c55#diff-7b17ff0818bb617e18a131fd82f5ff2bL438