`selectWith . pure` generates invalid SQL

Versions:

* `beam-core-0.10.1.0`
* `beam-postgres-0.5.3.1`

Code such as

```haskell
selectReturningList . selectWith . pure $ do
  all_ $ yourTable yourDatabase
```

generates invalid SQL.

A `WITH` clause requires at least 1 CTE, but `selectWith . pure` generates a query with 0 CTEs, which is a syntax error.
