<OnChange> without name attribute broken after react-final-form update

### Are you submitting a **bug report** or a **feature request**?

**Bug report** (minor update of a peer dependency, react-final-form (6.4.0 -> 6.5.0), causes breaking change).

### What is the current behavior?

When using an `` component without `name` attribute specified, it used to pass a whole values object to the `children` callback as an argument. But with a recent update of `react-final-form` from 6.4.0 to 6.5.0, the error started appearing:

```
prop name cannot be undefined in component
```

### What is the expected behavior?

The callback passed a whole object of values if no field name specified to listen to in `` component. See the screenshot with desired behavior below.


### Sandbox Link

Sandbox with desired behavior (notice an outdated `react-final-form` version, 6.4.0). Try typing in fields and see the console output.
https://codesandbox.io/s/affectionate-raman-r7qm4?file=/src/App.js&expanddevtools=1

Try changing the version to the latest, and error starts appearing.