minor: ReferencedSiteConfigurationImpl#getAccessUri() should return NULL if an empty string is configured as URI. This is because an empty string is the default for the ReferencedSite. It brings this method also in line with others similar getters such as getQueryUri()

git-svn-id: https://svn.apache.org/repos/asf/stanbol/trunk@1467926 13f79535-47bb-0310-9956-ffa450edef68