Fixed the readExternal method : if the value has an AttributeType with no Equality atchingRule, we were having NPE because we weren't testing this case.
This will fix DIRSERVER-1834

git-svn-id: https://svn.apache.org/repos/asf/directory/shared/trunk@1471294 13f79535-47bb-0310-9956-ffa450edef68