HBASE-27281 Add default implementation for Connection$getClusterId (#4683)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

Author: petersomogyi

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

@@ -171,8 +171,13 @@ default Table getTable(TableName tableName, ExecutorService pool) throws IOExcep
    */
   TableBuilder getTableBuilder(TableName tableName, ExecutorService pool);
 
-  /** Returns the cluster ID unique to this HBase cluster. */
-  String getClusterId();
+  /**
+   * Returns the cluster ID unique to this HBase cluster. <br>
+   * The default implementation is added to keep client compatibility.
+   */
+  default String getClusterId() {
+    return null;
+  }
 
   /**
    * Retrieve an Hbck implementation to fix an HBase cluster. The returned Hbck is not guaranteed to