Enforce overrinding execute() method in IntentUpdate subclasses

Change-Id: I02cf03abd60f771357b98bbd79c83eb3e0dce958
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java b/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
index a2ed4d9..df0b267 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/CompletedIntentUpdate.java
@@ -21,6 +21,7 @@
 
 import java.util.Collections;
 import java.util.List;
+import java.util.Optional;
 
 /**
  * Represents a completed phase of processing an intent.
@@ -63,4 +64,9 @@
     default List<Intent> allInstallables() {
         return Collections.emptyList();
     }
+
+    @Override
+    default Optional<IntentUpdate> execute() {
+        return Optional.empty();
+    }
 }