Database.Esqueleto.PostgreSQL.upsertBy syntax error in generated SQL

Error was found in esqueleto 3.5.3 from hackage. To reproduce, you need to apply expression similar to

```haskell
import qualified Database.Esqueleto.PostgreSQL as Psql
Psql.upsertBy
(UniqueUser email)
userData
[]
```

And then error will look like

```
uncaught exception: SomeAsyncException
ExceptionInLinkedThread ThreadId 3880 (SqlError {sqlState = "42601", sqlExecStatus = Fat
alError, sqlErrorMsg = "syntax error at or near \"WHERE\"", sqlErrorDetail = "", sqlErrorHint =
""})
```

Expected behaviour - atomic insert in case where Unique key was not used yet, and ignore (no update) in case Unique key has been already used.

There is a workaround to avoid such error and get expected behaviour - you need to do redundant update of Unique key like this:

```haskell
import qualified Database.Esqueleto.PostgreSQL as Psql
Psql.upsertBy
(UniqueUser email)
userData
[UserEmail =. val email]
```

But in my opinion it should work as expected without any workarounds.

Start with Database.Esqueleto.PostgreSQL.upsertBy and reproduce the reported call using an unused and an already-used UniqueUser key against PostgreSQL. Inspect the generated SQL around the WHERE clause; done means the call performs an atomic insert for a new key and ignores an existing key without requiring a redundant update.

Written by the indexing model from the issue text.