ALTER TABLE ADD JSON NOT NULL creates existing rows that violate the generated json_valid() CHECK constrain

Hi, MariaDB developer. I find a bug.

When adding a JSON NOT NULL column to a non-empty table, MariaDB completes the ALTER TABLE successfully but initializes existing rows with an empty string.

Since MariaDB represents JSON as LONGTEXT with an automatically generated CHECK (json_valid(column)) constraint, the resulting row immediately violates the table's own constraint.

Step to Rproduce

DROP TABLE IF EXISTS t0;

CREATE TABLE t0 (c0 INT, PRIMARY KEY (c0));

INSERT INTO t0 (c0) VALUES (1);

ALTER TABLE t0 ADD COLUMN c1 JSON NOT NULL;

SHOW CREATE TABLE t0;

-- c1 longtext NOT NULL CHECK (json_valid(`c1`))

SELECT CONSTRAINT_NAME, CHECK_CLAUSE

FROM information_schema.CHECK_CONSTRAINTS

WHERE CONSTRAINT_SCHEMA = DATABASE() AND TABLE_NAME = 't0';

--  MariaDB returns: c1 | json_valid(`c1`)

SELECT c0, c1, json_valid(c1) FROM t0;

-- MariaDB returns: 1 |  | 0

SELECT * FROM t0 WHERE (json_valid(c1)) IS FALSE;

-- MariaDB returns: 1 |  

-- MySQL returns empty result set

Expected Behavior

A successful ALTER TABLE should not leave existing rows violating the resulting schema.

MariaDB should either reject the operation or initialize existing rows with a valid JSON value.

Actual Behavior

The operation succeeds, but the existing row gets an empty string, for which:

json_valid(c1) = 0

Therefore, the table contains a row that violates its automatically generated CHECK (json_valid(c1)) constraint immediately after the DDL operation.