Removing last item from array doesn't make the form dirty

### What is the current behavior?

When I have an array of items, removing the last item doesn't make the form dirty. Therefor, if you use `pristine` to enable/disable the form submission button, it won't become enabled.

This was **not** the case in an older version (I'm not sure if the bug is in `final-form`, `final-form-arrays`, `react-final-form` or `react-final-form-arrays`).

### Steps to reproduce

1. Use the first CodeSandbox link
2. Hit the X mark at the last item
3. The Submit button doesn't become enabled, because the form is not dirty (pristine).

You can see that `pristine` on the bottom is `true`, which should be `false`,

### What is the expected behavior?

Removing the last item in an array will make the form dirty (`pristine` should become `false`). Use the second CodeSandbox as a reference.

### Sandbox Link

Unexpected behavior: https://codesandbox.io/s/14nqx1w807
Expected behavior: https://codesandbox.io/s/5m284838xn

### What's your environment?

OS: Windows 10 1809
Browsers: Mozilla Firefox 63.0.3 and Google Chrome 71.0.3578.80.

Versions where it works as expected:

**final-form:** 1.2.1
**final-form-arrays:** 1.0.0
**react:** 16.2.0
**react-dom:** 16.2.0
**react-final-form:** 1.1.1
**react-final-form-arrays:** 1.0.0

Versions where it does **not** work as expected:

**final-form:** 4.10.0
**final-form-arrays:** 1.1.0
**react:** 16.4.2
**react-dom:** 16.6.2
**react-final-form:** 3.6.7
**react-final-form-arrays:** 1.1.0

### Other information

As you can see, in the first CodeSandbox the Submit button becomes enabled (and you can also see that the `pristine` value is `false`), and in the second CodeSandbox, the Submit button doesn't become enabled (and you can also see that the `pristine` value is `true`).

You can see that the Submit button becomes enabled for a very short time and then it becomes disabled. In the first `render`, `pristine` is actually `false`, but in the second `render`, `pristine` becomes `true`, which disables the button.