Make possible to rebuild restapitests job

Context

We have the restapitests status on a PR which is a separate job triggered by the nuxeo job.

When this job failed, one must go to the Trigger REST API tests stage and click on Restart Trigger REST API tests to rebuild the downstream job.

We have no direct link on the PR to access this view, which is only accessible in Blue Ocean.

Going to the rest-api-compatibility-tests failing job and clicking on Rerun doesn't work correctly as the upstreamBuild is not available in the rerun context. This causes the status to be set on the rest-api-compatibility-tests repository instead of the nuxeo repository.

Request

We should get rid of the currentBuild.upstreamBuilds.isEmpty() call in the isTriggered method to have correct GitHub statuses and Jira comments when nuxeo repository triggers this job