
**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
The shallow renderer defaults a class component's state to empty object if it's not set. "Real" renderers don't seem to do this, so a component can render successfully in the shallow renderer, but fail at runtime in the browser with a null reference error.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.**
Here is ReactDOM's behavior (crashes reading an uninitialized state value): https://codesandbox.io/s/4jv82zmp0
Here is the shallow renderer behavior (component able to render): https://codesandbox.io/s/pp733nrxzq

**What is the expected behavior?**
They initialize state in the same way.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16. This wasn't an issue in React 15.
render() {
        return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    expect(() => shallowRenderer.render(SomeComponent)).toThrowError(
      'ReactShallowRenderer render(): Invalid component element. Instead of ' +
        'passing a component class, make sure to instantiate it by passing it ' +
        'to React.createElement.',
    );
    expect(() => shallowRenderer.render(<div />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with ' +
        'custom components, not primitives (div). Instead of calling ' +
        '`.render(el)` and inspecting the rendered output, look at `el.props` ' +
        'directly instead.',
    );
  });
  it('should have shallow unmounting', () => {
    const componentWillUnmount = jest.fn();
    class SomeComponent extends React.Component {
      componentWillUnmount = componentWillUnmount;
      render() {