v2.7.1

Fix bug with children that render null

Previously, we fixed an issue with null children:

<FlipMove>
  {null}
</FlipMove>

We hadn't accounted, however, for children that render null:

<FlipMove>
  <CustomComponent />
</FlipMove>

class CustomComponent extends Component {
  render() {
    return null;
  }
}

This patch addresses this second case, by avoiding doing any DOM manipulation for elements that don't return a DOM node.