Handle the case when the value of a resultMap entry is null

(cherry picked from commit effdedf2)

rest/rest-server/src/main/java/org/ow2/proactive_grid_cloud_portal/scheduler/SchedulerStateRest.java

@@ -376,8 +376,8 @@ public class SchedulerStateRest implements SchedulerRestInterface {
         }
         return source.entrySet()
                      .stream()
-                     .filter(entry -> entry.getValue() != null)
-                     .collect(Collectors.toMap(Entry::getKey, entry -> entry.getValue().toString()));
+                     .collect(Collectors.toMap(Entry::getKey,
+                                               entry -> entry.getValue() != null ? entry.getValue().toString() : ""));
 
     }