SLING-2944 Prevent NPE if there is no authenticationInfo

Improved fix: use an empty map if authenticationInfo is null. This limits
the number of null checks to one and the rest of processing can safely
assume there is a map

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1511779 13f79535-47bb-0310-9956-ffa450edef68