https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
expect(container.innerHTML).toBe('');
    expect(ops).toEqual([
      'Parent:b componentWillUnmount',
      'normal[0]:b componentWillUnmount',
      'portal1[0]:b componentWillUnmount',
      'normal[1]:b componentWillUnmount',
      'portal2[0]:b componentWillUnmount',
      'portal2[1]:b componentWillUnmount',
    ]);
  });
  it('should render nested portals', () => {
    const portalContainer1 = document.createElement('div');
    const portalContainer2 = document.createElement('div');
    const portalContainer3 = document.createElement('div');
    ReactDOM.render(
      [
        <div key="a">normal[0]</div>,
        ReactDOM.createPortal(
          [
            <div key="b">portal1[0]</div>,
            ReactDOM.createPortal(
              <div key="c">portal2[0]</div>,
              portalContainer2,
            ),
            ReactDOM.createPortal(
              <div key="d">portal3[0]</div>,
              portalContainer3,
            ),
            <div key="e">portal1[1]</div>,
          ],