make joinStrings null safe

bug5332

java/com.sap.sse.common/src/com/sap/sse/common/Util.java

@@ -571,7 +571,7 @@ public class Util {
             } else {
                 result.append(separator);
             }
-            result.append(object.toString());
+            result.append(object != null ? object.toString() : "null");
         }
         return result.toString();
     }