Fix exporting privileges of a MariaDB role - #20562

Description

MariaDB roles are listed in User accounts with an empty host name. Exporting one runs SHOW GRANTS FOR 'role'@'', which fails with error 1141 (There is no such grant defined for user 'role' on host ''), so the export is empty.

To reproduce on MariaDB:

CREATE ROLE r;
GRANT SELECT ON test.* TO r;

User accounts -> Export for r.

On MariaDB, an account with an empty host is now addressed without the host part (SHOW GRANTS FOR 'role'). A test is added.

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