Make Role optional in ReplicationConfiguration (#1268)

Signed-off-by: Bala.FA <bala@minio.io>

Author: balamurugana

api/src/main/java/io/minio/messages/ReplicationConfiguration.java

@@ -21,6 +21,7 @@
 import java.util.List;
 import java.util.Objects;
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import org.simpleframework.xml.Element;
 import org.simpleframework.xml.ElementList;
 import org.simpleframework.xml.Namespace;
@@ -36,21 +37,17 @@
 @Root(name = "ReplicationConfiguration")
 @Namespace(reference = "http://s3.amazonaws.com/doc/2006-03-01/")
 public class ReplicationConfiguration {
-  @Element(name = "Role")
+  @Element(name = "Role", required = false)
   private String role;
 
   @ElementList(name = "Rule", inline = true)
   private List<ReplicationRule> rules;
 
   /** Constructs new replication configuration. */
   public ReplicationConfiguration(
-      @Nonnull @Element(name = "Role") String role,
+      @Nullable @Element(name = "Role", required = false) String role,
       @Nonnull @ElementList(name = "Rule", inline = true) List<ReplicationRule> rules) {
-
-    this.role = Objects.requireNonNull(role, "Role must not be null");
-    if (role.isEmpty()) {
-      throw new IllegalArgumentException("Role must not be empty");
-    }
+    this.role = role; // Role is not applicable in MinIO server and it is optional.
 
     this.rules =
         Collections.unmodifiableList(Objects.requireNonNull(rules, "Rules must not be null"));