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 
container,
    );
    expect(portalContainer.innerHTML).toBe('<div>portal:2</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<p>portal:3</p>, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('<p>portal:3</p>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(['Hi', 'Bye'], portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('HiBye');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(['Bye', 'Hi'], portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('ByeHi');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(null, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('');
    expect(container.innerHTML).toBe('<div></div>');
  });
  it('should keep track of namespace across portals (simple)', () => {