allowNull={false} not working?

When a user backspaces until an input is blank, the value of the input inside of the `values` object becomes `null`. Unless I'm misunderstanding, this should never be possible due to the `allowNull` prop?

Demo: https://codesandbox.io/s/z6w7qkky83

For scanning
```
const initialValues = { lastName: '' };
const App = () => (
{
if (values.lastName == null) {
console.warn('-------lastName is null----------');
}
return (

);
}}
/>
);
```