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**
const e = await render(<div>{null}foo</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(1);
          expectTextNode(e.childNodes[0], 'foo');
        } else {
          
          expect(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], 'foo');
        }
      });
      itRenders('false children as blank', async render => {
        const e = await render(<div>{false}foo</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(1);
          expectTextNode(e.childNodes[0], 'foo');
        } else {
          
          expect(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], 'foo');
        }
      });
      itRenders('null and false children together as blank', async render => {
        const e = await render(<div>{false}{null}foo{null}{false}</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(1);
          expectTextNode(e.childNodes[0], 'foo');
        } else {