Expose empty annotations as SparseAnnotations to move down cast

Change-Id: I9e8a7d61b770faaec1b7e0ea1cf78bdb367b68ee

@@ -29,7 +29,7 @@ import static com.google.common.base.Preconditions.checkNotNull;
  */
 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;
 

@@ -465,7 +465,7 @@ class ConfigProvider implements DeviceProvider, LinkProvider, HostProvider {
     // Produces set of annotations from the given JSON node.
     private SparseAnnotations annotations(JsonNode node) {
         if (node == null) {
-            return (SparseAnnotations) DefaultAnnotations.EMPTY;
+            return DefaultAnnotations.EMPTY;
         }
 
         DefaultAnnotations.Builder builder = DefaultAnnotations.builder();