Add Collections.emptySet()

Change-Id: I51e845ed69f688a859089a221643fd6d04ac7afa
diff --git a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/KryoNamespaces.java b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/KryoNamespaces.java
index e29423c..b366dbf 100644
--- a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/KryoNamespaces.java
+++ b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/KryoNamespaces.java
@@ -153,6 +153,7 @@
             .register(new ArraysAsListSerializer(), Arrays.asList().getClass())
             .register(Collections.singletonList(1).getClass())
             .register(Duration.class)
+            .register(Collections.emptySet().getClass())
             .build();
 
     /**