Fixes errors when removing missing tables

1. link_deleted_tables was not filtering by current user,
   that could lead to tables being dropped on another user
   account.
2. remove tables with NULL table_id (oid).
   This query:
   ```sql
   select * from bar where foo = null;
   ```
   Is not correct, you have to:
   ```sql
   select * from bar where foo is null;
   ```