SLING-3663 : adding another potential fix : getModules was caching an empty array, which was likely due to timing issue at project-creation time. now clearing the cache when no modules are found for a project - which should not occur frequently

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1603164 13f79535-47bb-0310-9956-ffa450edef68