ERROR 1062 (23000): Duplicate entry 'NULL' for key 'key0' on CONCAT in sub-query

SELECT (SELECT CONCAT(partner.companyName, partner.lastName)

        FROM test_partner partner

        WHERE mvt.delivery_fk = partner.id) AS val

FROM test_movement mvt

WHERE mvt.documentId > 'a';

# Setup a clean mariadb

docker run --rm --name mariadb_test -d --env MARIADB_ALLOW_EMPTY_ROOT_PASSWORD=1 mariadb:11.6

# Create a test db

docker exec mariadb_test mariadb -e 'CREATE DATABASE test'

# Import schema

docker exec -i mariadb_test mariadb test < 01_schema.sql

# Import data

docker exec -i mariadb_test mariadb test < 02_data_partner.sql

docker exec -i mariadb_test mariadb test < 03_data_movement.sql

# Execute the query to trigger the bug

docker exec -i mariadb_test mariadb test < 04_query.sql

# Cleanup

docker stop mariadb_test