Removing parts of date fields

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

### What is the current behavior?
Removing part of a `` value ("yyyy-mm-dd") results in a thrown error. Such native elements don't normally allow such removal, so I don't know what's causing it.

### What is the expected behavior?
Removing parts of a date field should not be allowed.

### Sandbox Link
https://codesandbox.io/s/removing-parts-of-a-date-with-react-form-final-3hkze

### What's your environment?
final-form `4.18.5`
react `16.8.6`
react-dom `16.8.6`
react-final-form `6.3.0`
react-scripts `3.0.1`

Chrome `77.0.3865.120`
macOS `10.15`

### Other information