Tolerate null class list in Roles.yaml

puppet-classifier is currently failing for the default manifest because the
value of 'classes' when all roles are commented out is null (parsed by PyYAML
into None). This patch tolerates null and unset 'classes' in the YAML manifest.

Change-Id: Ie9c37b8bd2cebfbacb2761f1bad762aa2854587d