HBASE-27281 Add default implementation for Connection$getClusterId (a…

…pache#4683)

Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit b9a13eb)
Change-Id: I39d1ef6a0e5ff6e0f62e5a1fc1b3e4a26843afdb

hbase-client/src/main/java/org/apache/hadoop/hbase/client/Connection.java

@@ -172,9 +172,12 @@ default Table getTable(TableName tableName, ExecutorService pool) throws IOExcep
   TableBuilder getTableBuilder(TableName tableName, ExecutorService pool);
 
   /**
-   * @return the cluster ID unique to this HBase cluster.
+   * Returns the cluster ID unique to this HBase cluster. <br>
+   * The default implementation is added to keep client compatibility.
    */
-  String getClusterId();
+  default String getClusterId() {
+    return null;
+  }
 
   /**
    * Retrieve an Hbck implementation to fix an HBase cluster. The returned Hbck is not guaranteed to