#653 - Polishing.

Expose nullability in Identifiable, ResourceSupport and Links via Optional.

src/main/java/org/springframework/hateoas/Identifiable.java

@@ -16,6 +16,7 @@
 package org.springframework.hateoas;
 
 import java.io.Serializable;
+import java.util.Optional;
 
 /**
  * Interface to mark objects that are identifiable by an ID of any type.
@@ -27,7 +28,7 @@ public interface Identifiable<ID extends Serializable> {
 	/**
 	 * Returns the id identifying the object.
 	 * 
-	 * @return the identifier or {@literal null} if not available.
+	 * @return the identifier or {@link Optional#empty()} if not available.
 	 */
-	ID getId();
+	Optional<ID> getId();
 }