Change the return type of ResourceId#parent()

Change-Id: Ice8c46f4dca02cfc24f85a6758a8536872b4c262
diff --git a/core/api/src/main/java/org/onosproject/net/newresource/DiscreteResource.java b/core/api/src/main/java/org/onosproject/net/newresource/DiscreteResource.java
index e88a2a3..c8839a6 100644
--- a/core/api/src/main/java/org/onosproject/net/newresource/DiscreteResource.java
+++ b/core/api/src/main/java/org/onosproject/net/newresource/DiscreteResource.java
@@ -90,7 +90,7 @@
 
     @Override
     public Optional<DiscreteResource> parent() {
-        return Optional.ofNullable(id.parent()).map(DiscreteResource::new);
+        return id.parent().map(DiscreteResource::new);
     }
 
     @Override