Merge branch 'fix-empty-groups' into 'master'

Groups should return a value event if no process is active in the group

See merge request ongresinc/pgio!2

main/src/main/java/com/ongres/pgio/main/stats/ProcessGroupStat.java

@@ -61,11 +61,12 @@ public class ProcessGroupStat {
   
   private Optional<UnsignedLong> reduceFromStats(
       Function<ProcessStat, Optional<UnsignedLong>> valueSupplier) {
-    return stats.stream()
+    return Optional.of(stats.stream()
         .reduce(Optional.<UnsignedLong>empty(), 
             (result, stat) -> valueSupplier.apply(stat)
             .map(value -> result.orElse(UnsignedLong.ZERO).plus(value)), 
-            (u, v) -> v);
+            (u, v) -> v)
+        .orElse(UnsignedLong.ZERO));
   }
   
   @Override