WW-3990 Returns null when trying to escape null value instead of returning an empty string

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/trunk@1452255 13f79535-47bb-0310-9956-ffa450edef68

core/src/main/java/org/apache/struts2/components/UIBean.java

@@ -881,7 +881,7 @@ public abstract class UIBean extends Component {
         if (name != null) {
             return name.replaceAll("[\\/\\.\\[\\]]", "_");
         } else {
-            return "";
+            return null;
         }
     }
 
@@ -895,7 +895,7 @@ public abstract class UIBean extends Component {
         if (val != null) {
             return val.replaceAll("\"", """);
         } else {
-            return "";
+            return null;
         }
     }