Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=53198
driverClassName should not be mandatory
https://issues.apache.org/bugzilla/show_bug.cgi?id=53198

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1613066 13f79535-47bb-0310-9956-ffa450edef68