Ensure UpdateEntry comprison works well with ECMaps that have tombstones disabled

Change-Id: Iff3f09a8342a7d9e89a31fb2be86ca1bfa4d75ff
diff --git a/core/store/dist/src/main/java/org/onosproject/store/ecmap/UpdateEntry.java b/core/store/dist/src/main/java/org/onosproject/store/ecmap/UpdateEntry.java
index 2a607bb..53683b9 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/ecmap/UpdateEntry.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/ecmap/UpdateEntry.java
@@ -61,7 +61,7 @@
      * @return true if this entry is newer; false otherwise
      */
     public boolean isNewerThan(UpdateEntry<K, V> other) {
-        return other == null || value.isNewerThan(other.value);
+        return other == null || other.value == null || (value != null && value.isNewerThan(other.value));
     }
 
     @Override