[pg] Fix INTERNAL ERROR for psql \d command - list tables

[rekby]

Reproduce:

1. Run local_ydb with postgres experiment enabled https://github.com/ydb-platform/ydb/wiki/Local-run-postgres-tests
2. Connect to YDB with psql: psql postgres://root:1234@localhost:5432/local
3. echo '\d' | psql postgres://root:1234@localhost:5432/local

Query from psql

SELECT n.nspname as "Schema",
  c.relname as "Name",
  CASE c.relkind WHEN 'r' THEN 'table' WHEN 'v' THEN 'view' WHEN 'm' THEN 'materialized view' WHEN 'i' THEN 'index' WHEN 'S' THEN 'sequence' WHEN 's' THEN 'special' WHEN 't' THEN 'TOAST table' WHEN 'f' THEN 'foreign table' WHEN 'p' THEN 'partitioned table' WHEN 'I' THEN 'partitioned index' END as "Type",
  pg_catalog.pg_get_userbyid(c.relowner) as "Owner"
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
     LEFT JOIN pg_catalog.pg_am am ON am.oid = c.relam
WHERE c.relkind IN ('r','p','v','m','S','f','')
      AND n.nspname <> 'pg_catalog'
      AND n.nspname !~ '^pg_toast'
      AND n.nspname <> 'information_schema'
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 1,2;

Error

Status: INTERNAL_ERROR
Issues:
<main>: Fatal: ydb/library/yql/parser/pg_wrapper/comp_factory.cpp:456: Condition violated: `f', code: 1
<main>: Error: Query invalidated on scheme/internal error., code: 2019

[vitstn]

with PR:

local=> \d
Did not find any relations.