6614556: null location for MonitorContendedEnterEvent

Reviewed-by: jjh

src/share/classes/com/sun/tools/jdi/EventSetImpl.java

@@ -208,7 +208,8 @@ public class EventSetImpl extends ArrayList<Event> implements EventSet {
         }
 
         public String toString() {
-            return eventName() + "@" + location().toString() +
+            return eventName() + "@" +
+                   ((location() == null) ? " null" : location().toString()) +
                    " in thread " + thread().name();
         }
     }