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 and async examples do work successfully with this change.

This allows to simplify the code in Mapper.internalMap() by removing a number of checks.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1604434 13f79535-47bb-0310-9956-ffa450edef68