return default empty set for saved refs

otherwise hydrated refs don't hydrate their children

server/src/main/java/com/usatiuk/dhfs/storage/objects/repository/distributed/ObjectMetadata.java

@@ -31,7 +31,7 @@ public class ObjectMetadata implements Serializable {
 
     @Getter
     @Setter
-    private Set<String> _savedRefs = null;
+    private Set<String> _savedRefs = Collections.emptySet();
 
     @Getter
     private long _refcount = 0L;