Workaround for 3.6.1+ class evaluation bug

This is a workaround for a bug in 3.6.1+ (https://dev.cfengine.com/issues/6661) that prevents the services_array_${services} lists from being properly dereferenced. The absence of the check does not hurt execution because the checks in the methods: and reports: section will catch the case in which @(services) is not defined anyway.