JSON_ARRAYAGG does not skip NULL by default

The standard for `JSON_ARRAYAGG` has an implicit default value of `ABSENT ON NULL`. MariaDB does not implicitly apply this option, and applies `NULL ON NULL` as the incorrect default:

See:

• MSSQL - https://learn.microsoft.com/en-us/sql/t-sql/functions/json-arrayagg-transact-sql?view=azuresqldb-current
• Oracle - https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/JSON_ARRAYAGG.html
• IBM - https://www.ibm.com/docs/en/i/7.5?topic=functions-json-arrayagg

Note, MySQL is also has a bugged approach:

• MySQL (also bugged) https://bugs.mysql.com/bug.php?id=90835

The immediate fix is to just change this to true.

bool skip_nulls() const override { return false; }

https://github.com/MariaDB/server/blob/ecc7961140968b3a9d464b65786bb3a3ca92bb42/sql/item_jsonfunc.h#L716

Adding support for `NULL_ON_NULL` and `ABSENT_ON_NULL` could be added on separately.