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 
render() {
        return <div>Foo</div>;
      }
    }
    const element = <Foo />;
    
    let called = false;
    ReactDOM.render(element, container, () => (called = true));
    expect(called).toEqual(true);
    
    called = false;
    ReactDOM.unstable_batchedUpdates(() => {
      ReactDOM.render(element, container, () => (called = true));
    });
    expect(called).toEqual(true);
  });
  it('should render a component returning strings directly from render', () => {
    const Text = ({value}) => value;
    ReactDOM.render(<Text value="foo" />, container);
    expect(container.textContent).toEqual('foo');
  });
  it('should render a component returning numbers directly from render', () => {
    const Text = ({value}) => value;
    ReactDOM.render(<Text value={10} />, container);
    expect(container.textContent).toEqual('10');
  });
  it('finds the DOM Text node of a string child', () => {
    class Text extends React.Component {