DERBY-6663: NPE when a trigger tries to insert into a table with a foreign key

Make sure FKInfo serializes its fkIds field so that the information is
available in stored prepared plans too.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1610711 13f79535-47bb-0310-9956-ffa450edef68