Return null instead of N/A on statistics

src/main/java/eu/dnetlib/repo/manager/service/StatsServiceImpl.java

@@ -97,7 +97,7 @@ public class StatsServiceImpl implements StatsService {
         stats.put("lastYearUsagestats", lastYearUsagestats);
 
 //        Integer usagestats = getUsageStatsTotal();
-        stats.put("usagestats","N/A");
+        stats.put("usagestats",null);
         return stats;
     }