VALUES produces invalid SQLite query

This example:

```haskell
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}

module Main where

import GHC.Generics
import Data.Text (Text)
import Database.Beam
import Database.Beam.Sqlite
import Database.Beam.Backend.SQL
import Database.Beam.Backend.Types
import Database.SQLite.Simple as SQLite
import Database.SQLite.Simple.FromField

data TestDb f
= TestDb
{ _tdbUsers :: f (TableEntity UserT)
}
deriving (Generic)

instance Database be TestDb

testDb :: DatabaseSettings be TestDb
testDb = defaultDbSettings

type Username = Text

mkUsername = id

--newtype Username = Username Text
-- deriving (Show, Eq, Ord)
--
--deriving instance HasSqlValueSyntax be Text => HasSqlValueSyntax be Username
--deriving instance FromField Username
--instance FromBackendRow Sqlite Username
--mkUsername :: Text -> Username
--mkUsername = Username

data UserT f
= User
{ _userId :: Columnar f Int
, _userName :: Columnar f Username
}
deriving (Generic)

type User = UserT Identity
type UserId = PrimaryKey UserT Identity
deriving instance Show (PrimaryKey UserT Identity)
deriving instance Show User

instance Table UserT where
data PrimaryKey UserT f = UserId (Columnar f Int) deriving Generic
primaryKey = UserId . _userId

instance Beamable UserT
instance Beamable (PrimaryKey UserT)

missingUsers :: [Username]
-> Q Sqlite TestDb s (QGenExpr QValueContext Sqlite s Username)
missingUsers users =
let userNames :: Q Sqlite TestDb s (QGenExpr QValueContext Sqlite s Username)
userNames = fmap _userName (all_ $ _tdbUsers testDb)

userNames' :: Q Sqlite TestDb s (QGenExpr w Sqlite s Username)
userNames' = values_ $ map val_ users
in userNames `except_` userNames'

main :: IO ()
main = do
conn <- SQLite.open "test.sqlite"
let users = map mkUsername ["a", "b", "c"]
runBeamSqliteDebug putStrLn conn $
runSelectReturningList (select $ missingUsers users) >>= liftIO . print
```

Fails with:
```
SQLite3 returned ErrorError while attempting to perform prepare "SELECT \"t0\".\"name\" AS \"res0\" FROM \"users\" AS \"t0\" EXCEPT SELECT \"t0\".\"res0\" AS \"res0\" FROM (VALUES (?), (?), (?)) AS \"t0\"(\"res0\")": near "(": syntax error
```