Stuck at inserting records into MariaDB conditionally

I know about insert ignore. That's of no use, because MariaDB does not consider NULL to be part of the uniqude index. Therefore I have to use if ... else. But this code does not run:

select *
from Nodes 
where Key = 'FAQPage';

if not found_row() then
   insert into Nodes (Key, ParentId)
   values ('FAQPage', null)
end if;

How can I use if ... else to conditionally insert data in MariaDB?