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**
}
      });
    });
    describe('null, false, and undefined children', function() {
      itRenders('null single child as blank', async render => {
        const e = await render(<div>{null}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('false single child as blank', async render => {
        const e = await render(<div>{false}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('undefined single child as blank', async render => {
        const e = await render(<div>{undefined}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('a null component children as empty', async render => {
        const NullComponent = () => null;
        const e = await render(<div><NullComponent /></div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(0);
        } else {
          
          
          expect(e.childNodes.length).toBe(1);
          expectEmptyNode(e.firstChild);
        }
      });
      itRenders('null children as blank', async render => {