Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56522
When setting a value for a ValueExpression, ensure that the expected coercions take place such as a null string being coerced to an empty string.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1594723 13f79535-47bb-0310-9956-ffa450edef68