#1427: Prevent job triggering for non-matching webhooks

[mc1arke]

8ec69c7ada9da38a760f532b9dd34bdf538f8ccb introduced a change that does not handle jobs created using Jenkins DSL properly, as DSL jobs that do not specify triggerOpenMergeRequestOnPush have the triggerOpenMergeRequest value passed into MergeRequestHookTriggerHandlerFactory as null, with the above change causing the value not to be handled as never where the plugin previously treated null as never. This results in webhooks triggering builds for DSL-created jobs even where the job is doing actions that aren't valid for a Merge Request in that state (e.g. triggering release builds even though the Merge Request hasn't been merged). This change returns to handling an unspecified triggerOpenMergeRequest value as being equivalent to specifying it as never.

Testing done

Automated test to cover failing scenario.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[krisstern]

Thanks @mc1arke for the PR! I will review it over the next few days

[krisstern]

Looks like you may want to run mvn spotless:apply to fix the remaining failed checks

[krisstern]

I will cut a release with this code change within the next 24 hours.

[krisstern]

New release cut at https://github.com/jenkinsci/gitlab-plugin/releases/tag/gitlab-plugin-1.9.6