Fix list functions column type error on scan

This is a similar issue/fix as https://github.com/xo/usql/issues/382 with `routine_definition` when trying to list functions (in a postgres database): ``` Connected with driver postgres (PostgreSQL 16.4) Type "help" for help. pg:my_postgres_dsn=> \df error: failed to list functions: sql: Scan error on column index 6, name "routine_definition": converting NULL to string is unsupported ```