title: Redux form doesn't pass new value when the value change from null to empty string
body: <p>I have a custom select component which has a different behavior if it's value is null or an empty string.</p>
  
  <p>To be able to pass a null value to the Field component, I put <code>format={null}</code> as per <a href="https://redux-form.com/7.4.2/docs/api/field.md/" rel="nofollow noreferrer">the documentation</a>.</p>
  
  <p>In the redux store, my component is correctly initialized with <code>value: null</code> and the component <code>input.value</code> is also <code>null</code>, as expected. When I select the value with the empty string, redux-form correctly dispatch the event <code>@@redux-form/CHANGE</code> and the value of my Field correctly change from <code>null</code> to <code>''</code> but my <code>input.value</code> in my component never change and is still <code>null</code>.</p>
  
  <p>On the other hand, if I select a non null &amp; non empty string value, then the empty string value (e.g <code>null</code> => <code>'1'</code> => <code>''</code>), the <code>input.value</code> is correctly updated and my component correctly render. </p>
  
  <p>Does someone know why doing <code>null</code> => <code>''</code> doesn't change the <code>input.value</code> but <code>null</code> => <code>'a value'</code> => <code>''</code> does ?</p>