Incorrect pristine value

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

A bug, pristine value is incorrect when filling a field, then emptying it.

### What is the current behavior?

Initial render => the form is pristine
I fill a field => the form is dirty
I empty the field => the form is still dirty

### What is the expected behavior?

Initial render => the form is pristine
I fill a field => the form is dirty
I empty the field => the form is pristine again

### Sandbox Link

Works as intended in RFF but not in RFFH:

RFF : https://zn82m7zlrp.codesandbox.io/
vs
RFFH : https://71y11y0v1.codesandbox.io/

### What's your environment?

final-form : 4.12.0
react-final-form-hoooks : 1.0.1

### Other information

Thank you for all your awesome work :)