Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56665
Merged revision 1605402 from tomcat/trunk:
Correct the generation of the effective web.xml when elements contain an empty string as value.


git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1605405 13f79535-47bb-0310-9956-ffa450edef68