Fix list functions column null error on scan

[remisalmon]

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