[ONOS-2499]update accordingly for ONOS-2408's review recommendation

Change-Id: I24e7c4acbe38e4286b994c5bf50d22def945fb29
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/message/TableUpdate.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/message/TableUpdate.java
index 5228490..812a034 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/message/TableUpdate.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/message/TableUpdate.java
@@ -16,6 +16,7 @@
 package org.onosproject.ovsdb.rfc.message;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
+import static com.google.common.base.Preconditions.checkNotNull;
 
 import java.util.Map;
 import java.util.Objects;
@@ -44,6 +45,7 @@
      * @return TableUpdate entity
      */
     public static TableUpdate tableUpdate(Map<UUID, RowUpdate> rows) {
+        checkNotNull(rows, "rows cannot be null");
         return new TableUpdate(rows);
     }