Assert that MappingData object is empty before performing mapping work.
The old code just skipped all work in that case, but I do not see why that would be useful,
and it may return inconsistent results.
The comet, async, websocket examples do work successfully with this change.
It is backport of r1604434 from tomcat/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1604663 13f79535-47bb-0310-9956-ffa450edef68