Conditionally-rendered FieldArray does not render children if fields.push() is called on initialise

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

### What is the current behavior?
Conditionally-rendered FieldArrays do not re-render when `fields.push()` is called manually.

https://codesandbox.io/s/react-final-form-field-arrays-28hnd/

### What is the expected behavior?
`fields.push()` should result in `fields` prop changing, and a re-render occurring.

### Sandbox Link
Type in the Company field to render the FieldArray. Delete what you typed, and type something again, and it will work as it should.
https://codesandbox.io/s/react-final-form-field-arrays-28hnd/

### What's your environment?
Final Form 4.18.6
Final Form Arrays 3.0.2
React Final Form 6.3.3
React Final Form Arrays 3.1.1