Another m1.0.2 compatibility issue

git-svn-id: https://svn.apache.org/repos/asf/maven/maven-1/plugins/trunk@354501 13f79535-47bb-0310-9956-ffa450edef68

xdoc/src/plugin-resources/sitemap.jsl

@@ -72,7 +72,7 @@
 
     <jsl:template match="item">
       <x:set var="currentItem" select="." />
-      <x:set var="description" select="null"/>
+      <j:set var="description" value=""/>
       <j:set var="link"><x:expr select="@href"/></j:set>
       <j:if test="${link.startsWith('/')}" trim="yes">
         <j:set var="link" value="${link.substring(1)}"/>