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 
const ReactDOM = require('react-dom');
const ReactTestUtils = require('react-dom/test-utils');
const PropTypes = require('prop-types');
describe('ReactDOMFiber', () => {
  let container;
  beforeEach(() => {
    container = document.createElement('div');
  });
  it('should render strings as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value="foo" />, container);
    expect(container.textContent).toEqual('foo');
  });
  it('should render numbers as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value={10} />, container);
    expect(container.textContent).toEqual('10');
  });
  it('should be called a callback argument', () => {
    
    let called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
    
    called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
  });
  it('should call a callback argument when the same element is re-rendered', () => {
    class Foo extends React.Component {