JenkinsService test fails when no current license present

Trying to test the Jenkins service causes this:

It looks like this is because the JenkinsService#execute returns nil if the license doesn't cover Jenkins:

• https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/models/project_services/jenkins_service.rb#L31
• https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/ee/project.rb#L484-493

/cc: @rdavila

Zendesk: https://gitlab.zendesk.com/agent/tickets/81390