I'm using MariaDB 10.5.1-MariaDB-1:10.5.1+maria~bionic on Docker for Windows. The following query crashes the server:

select json_arrayagg(null);

All sorts of alternative ways to aggregate null lead to the same effect:

select json_arrayagg((select null));

select json_arrayagg(a) from (select null a) t;

But if there are more than 1 null values, the result is as expected:

select json_arrayagg(a) from (select null a union all select null) t;

Yields

[null, null]