[ONOS-6874] Implement nullable ConsistentMap

Change-Id: I82a232d376b230b06977b24f12364aa374c9c113
diff --git a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/DefaultConsistentMapBuilder.java b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/DefaultConsistentMapBuilder.java
index 32a7506..2bc06dd 100644
--- a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/DefaultConsistentMapBuilder.java
+++ b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/DefaultConsistentMapBuilder.java
@@ -42,6 +42,7 @@
     @Override
     public AsyncConsistentMap<K, V> buildAsyncMap() {
         AsyncConsistentMap<K, V> map = primitiveCreator.newAsyncConsistentMap(name(), serializer());
+        map = nullValues() ? map : DistributedPrimitives.newNotNullMap(map);
         map = relaxedReadConsistency() ? DistributedPrimitives.newCachingMap(map) : map;
         map = readOnly() ? DistributedPrimitives.newUnmodifiableMap(map) : map;
         return meteringEnabled() ? DistributedPrimitives.newMeteredMap(map) : map;