MySQL MyISAM query still optimized when the first column is null - mysql

Question

In MySQL offcial document for count,there are descriptions as below:
For MyISAM tables, COUNT(*) is optimized to return very quickly if the SELECT retrieves from one > table, no other columns are retrieved, and there is no WHERE clause. For example:
mysql> SELECT COUNT(*) FROM student;
This optimization only applies to MyISAM tables, because an exact row count is stored for this >storage engine and can be accessed very quickly. COUNT(1) is only subject to the same >optimization if the first column is defined as NOT NULL.
I want to test it for myself, and make a test as below,I make a table called system_user and the first column is type,all the value of type is null,however when I use SELECT COUNT(1) and SELECT COUNT(*) to query, I found the time cost is nearly the same,even if tried for serval times.
I am wonder why the first column is null and the optimization in MyISAM is still working?
When I use SQL_NO_CACHE,the time cost is still nearly the same:
related table:
CREATE TABLE `system_user` (
`type` varchar(10) DEFAULT NULL,
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(8) NOT NULL,
`age` int(11) DEFAULT NULL,
`tag` varchar(8) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
related procdure to create massive data:
DELIMITER $$
USE `test`$$
DROP PROCEDURE IF EXISTS `add_user_batch`$$
CREATE DEFINER=`root`#`%` PROCEDURE `add_user_batch`(IN COUNT INT)
BEGIN
DECLARE i INT;
DECLARE t_name VARCHAR(8);
DECLARE t_tag VARCHAR(20);
DECLARE t_age INT(2);
DECLARE t_sql_template VARCHAR(100);
DECLARE t_sql TEXT;
DECLARE t_tag_mod_val INT DEFAULT(25);
DECLARE t_commit_mod_val INT DEFAULT(100);
DECLARE t_start_time DATETIME;
DECLARE t_end_time DATETIME;
TRUNCATE TABLE `system_user`;
SET t_start_time=NOW();
SET t_sql_template = “INSERT INTO `system_user`(NAME, age, tag) VALUES“;
SET t_sql = t_sql_template;
SET i = 1;
WHILE i <= COUNT
DO
SET t_age = FLOOR(1 + RAND() * 60);
SET t_name = LEFT(UUID(), 8);
IF MOD(i, t_tag_mod_val) = 0 THEN
SET t_tag = “NULL“;
ELSE
SET t_tag = CONCAT(“'“,LEFT(UUID(), 8),“'“);
END IF;
SET t_sql = CONCAT(t_sql,“('“,t_name,“',“,t_age,“,“,t_tag,“)“);
IF MOD(i,t_commit_mod_val) != 0 THEN
SET t_sql = CONCAT(t_sql,“,“);
ELSE
SET t_sql = CONCAT(t_sql,“;“);
SET #insert_sql = t_sql;
PREPARE stmt FROM #insert_sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
COMMIT;
SET t_sql=t_sql_template;
END IF;
SET i = i + 1;
END WHILE;
IF LENGTH(t_sql) > LENGTH(t_sql_template) THEN
SET t_sql=CONCAT(SUBSTRING(t_sql,1,LENGTH(t_sql)-1),';');
SET #insert_sql = t_sql;
PREPARE stmt FROM #insert_sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
COMMIT;
END IF;
SET t_end_time=NOW();
SELECT CONCAT('insert data success,time cost ',TIMEDIFF(t_end_time,t_start_time)) AS finishedTag;
END$$
DELIMITER ;