FieldArray validate does not work with an initial row in initialValues

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

### What is the current behavior?

The current behavior is that when there are existing rows in `initialValues` the `FieldArray`s `validate` function when returning `[undefined]` impacts the field level validation. Interestingly, this does not affect any new rows.

See: https://codesandbox.io/s/react-final-form-field-arrays-forked-bqtle?file=/index.js

The field level validation checks if the value === `bad`. You will notice that typing `bad` in the first row does not result in an error but in any new rows it does work. The `FieldArray`'s `validate` is simply: `() => [undefined]`

### What is the expected behavior?

FieldArray validation should not impact Field level validation. This worked in:

* final-form 4.17.0
* final-form-arrays 2.0.0
* react-final-form 4.1.0
* react-final-form-arrays 2.0.3

but fails in:

* final-form 4.18.0
* final-form-arrays 3.0.0
* react-final-form 6.0.0
* react-final-form-arrays 3.0.0

### Sandbox Link

https://codesandbox.io/s/react-final-form-field-arrays-forked-bqtle?file=/index.js

### What's your environment?

This worked in:

* final-form 4.17.0
* final-form-arrays 2.0.0
* react-final-form 4.1.0
* react-final-form-arrays 2.0.3

but fails in:

* final-form 4.18.0
* final-form-arrays 3.0.0
* react-final-form 6.0.0
* react-final-form-arrays 3.0.0

### Other information