React 16 alpha allows returning array in render function but server renderer expects children to always be react elements

```
TypeError: Cannot read property 'toLowerCase' of undefined
    at ReactDOMServerRenderer.renderDOM
```

source
```javascript
function renderDOM(element, context) {
    var tag = element.type.toLowerCase(); // element is array
```

Fiddle: https://jsfiddle.net/84v837e9/160/

**react@16.0.0-alpha.13**
render() {
          return null;
        }
      }
      var response = ReactDOMServer.renderToString(<NullComponent />);
      if (ReactDOMFeatureFlags.useFiber) {
        expect(response).toBe('');
      } else {
        expect(response).toBe('<!-- react-empty: 1 -->');
      }
    });
    
    it('should render composite components', () => {
      class Parent extends React.Component {
        render() {
          return <div><Child name="child" /></div>;
        }
      }
      class Child extends React.Component {