Fix compile

Author: thecoop

server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java

@@ -171,7 +171,7 @@ public enum XContentContext {
         EnumSet.of(ClusterBlockLevel.WRITE, ClusterBlockLevel.METADATA_WRITE)
     );
 
-    public static final String EMPTY_METADATA = builder().build();
+    public static final Metadata EMPTY_METADATA = builder().build();
 
     public static final String CONTEXT_MODE_PARAM = "context_mode";