Make resource retrieval more efficient when specifing resource type

This resolves ONOS-4666

Change-Id: I9d09b60531ca48b36fc20f43498cda62f1badb8b
diff --git a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EmptyDiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EmptyDiscreteResources.java
index d7c0f24..55a0e94 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EmptyDiscreteResources.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EmptyDiscreteResources.java
@@ -68,4 +68,9 @@
                 .add("values", ImmutableSet.of())
                 .toString();
     }
+
+    @Override
+    public <T> Set<DiscreteResource> valuesOf(Class<T> cls) {
+        return ImmutableSet.of();
+    }
 }