GitHUB PR job should use when clause to filter to just PRs.

                        # If CHANGE_URL is set (e.g., Github Branch Source plugin), process it.
                        # Otherwise exit, because we don't want HBase to do a
                        # full build.  We wouldn't normally do this check for smaller
                        # projects. :)
                        if [[ -z "${CHANGE_URL}" ]]; then
                            echo "Full build skipped" > "${WORKSPACE}/${PATCHDIR}/report.html"
                            exit 0
                        fi

  stage ('precommit-run') {
    when { changeRequest() }
    steps {
      ...
    }
  }