-------------------+

SELECT CONCAT('Ma', 'ria', NULL, 'DB');
+---------------------------------+
| CONCAT('Ma', 'ria', NULL, 'DB') |
+---------------------------------+
| NULL                            |
+---------------------------------+

SELECT CONCAT(42.0);
+--------------+
| CONCAT(42.0) |
+--------------+
| 42.0         |
+--------------+

Using IFNULL() to handle NULLs:

SELECT CONCAT('The value of @v is: ', IFNULL(@v, ''));
+------------------------------------------------+
| CONCAT('The value of @v is: ', IFNULL(@v, '')) |
+------------------------------------------------+
| The value of @v is:                            |
+------------------------------------------------+

In Oracle mode, from MariaDB 10.3:

SELECT CONCAT('Ma', 'ria', NULL, 'DB');
+---------------------------------+
| CONCAT('Ma', 'ria', NULL, 'DB') |
+---------------------------------+
| MariaDB                         |
+---------------------------------+

URL: https://mariadb.com/kb/en/concat/https://mariadb.com/kb/en/concat/