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('<div>bar</div>');
    
    container.innerHTML = '<div>MEOW.</div>';
    expect(() => ReactDOM.render(<div>baz</div>, container)).toWarnDev(
      'render(...): ' +
        'It looks like the React-rendered content of this container was ' +
        'removed without using React. This is not supported and will ' +
        'cause errors. Instead, call ReactDOM.unmountComponentAtNode ' +
        'to empty a container.',
    );
  });
  it('should warn when doing an update to a container manually cleared outside of React', () => {
    
    ReactDOM.render(<div>foo</div>, container);
    ReactDOM.render(<div>bar</div>, container);
    expect(container.innerHTML).toBe('<div>bar</div>');
    
    container.innerHTML = '';
    expect(() => ReactDOM.render(<div>baz</div>, container)).toWarnDev(
      'render(...): ' +
        'It looks like the React-rendered content of this container was ' +
        'removed without using React. This is not supported and will ' +
        'cause errors. Instead, call ReactDOM.unmountComponentAtNode ' +
        'to empty a container.',
    );
  });
  it('should render a text component with a text DOM node on the same document as the container', () => {