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 
click(node);
    expect(ops).toEqual(['B']);
    ops = [];
    
    
    
    
    ReactDOM.render(
      [<Example key="a" forceA={true} />, <Click key="b" />],
      container,
    );
    
    
    expect(ops).toEqual(['B']);
    ops = [];
    
    click(node);
    expect(ops).toEqual(['A']);
  });
  it('should not crash encountering low-priority tree', () => {
    ReactDOM.render(
      <div hidden={true}>
        <div />
      </div>,
      container,
    );
  });
  it('should not warn when rendering into an empty container', () => {
    ReactDOM.render(<div>foo</div>, container);
    expect(container.innerHTML).toBe('<div>foo</div>');