Expose empty annotations as SparseAnnotations to move down cast

Change-Id: I9e8a7d61b770faaec1b7e0ea1cf78bdb367b68ee
diff --git a/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java b/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
index 3911d0e..4215d65 100644
--- a/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
+++ b/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
@@ -29,7 +29,7 @@
  */
 public final class DefaultAnnotations implements SparseAnnotations {
 
-    public static final Annotations EMPTY = DefaultAnnotations.builder().build();
+    public static final SparseAnnotations EMPTY = DefaultAnnotations.builder().build();
 
     private final Map<String, String> map;