[ONOS-6324] Add getOrDefault method to ConsistentMap.

Change-Id: Ice7ad6260c6eb8076320ef469874c0c4ceeadc19
diff --git a/core/api/src/main/java/org/onosproject/store/service/Versioned.java b/core/api/src/main/java/org/onosproject/store/service/Versioned.java
index 6c6834e..7de5be7 100644
--- a/core/api/src/main/java/org/onosproject/store/service/Versioned.java
+++ b/core/api/src/main/java/org/onosproject/store/service/Versioned.java
@@ -97,7 +97,7 @@
      * @return mapped instance
      */
     public synchronized <U> Versioned<U> map(Function<V, U> transformer) {
-        return new Versioned<>(transformer.apply(value), version, creationTime);
+        return new Versioned<>(value != null ? transformer.apply(value) : null, version, creationTime);
     }
 
     /**