No instance for (AllCTRender '[] ())

I'm getting the following error:

```
No instance for (Servant.API.ContentTypes.AllCTRender '[] ()) …
arising from a use of ‘serveWithContext’
```

when using API types such as `Post '[] ()` as in the tutorial examples.

I'm able to solve this with:

```
instance AllCTRender '[] () where
handleAcceptH _ _ _ = Nothing
```

but then I get this warning:

```
Warning: …
Orphan instance: instance AllCTRender '[] ()
```

Shouldn't this instance already be defined in `Servant.API.ContentTypes`?