Form level validation trumps field level validation when it should not

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

Bug report

### What is the current behavior?

When a field inside a `` is validated both at form level (using ``) and at field level (using ``), field level validation is always ignored, even when form level validation returns `undefined` for a given field in the ``.

### What is the expected behavior?

If one of the validators (form or field level) returns an error for a field in a field array, the form should be rendered invalid.

### Sandbox Link

https://codesandbox.io/s/stoic-fog-rcxru?file=/src/App.js

There should be a "required" error rendered next to the input, but instead no error is rendered. Removing the `validate` prop from `` fixes this and the "required" error gets rendered.

### What's your environment?

```
"final-form": "4.19.1",
"final-form-arrays": "3.0.2",
"react": "16.12.0",
"react-dom": "16.12.0",
"react-final-form": "6.4.0",
"react-final-form-arrays": "3.1.1",
```

### Other information

The bug only seems to affect fields that are present in `initialValues`. When a field is added to the array after mount, the newly added field's validation seems to behave as expected.