Support SELECTs without explicit table

Starting with the following postgres statement:

```sql
SELECT TRUE as can_has_cheese_burger
```

when I convert it to the db2 dialect, I get the following:

```sql
SELECT
  1 AS can_has_cheese_burger
```

but I think it should be more along the lines of:

```sql
SELECT
  1 AS can_has_cheese_burger
FROM "SYSIBM".SYSDUMMY1 s
```