[pg] Fix select from pg_tables by tablename

[rekby]

Reproduce:

1. https://github.com/ydb-platform/ydb/wiki/Local-run-postgres-tests
2. Execute

➤ echo "select tablename from pg_tables where tablename='pg_proc'" | psql postgres://root:1234@localhost:5432/local
Status: GENERIC_ERROR
Issues:
<main>: Error: Type annotation, code: 1030
    <main>:1:1: Error: At function: ResFill!
        <main>:1:1: Error: Mismatch of fields in hint and in the struct, columns fields: 1, struct fields:8