React.addons.TransitionGroup should not throw on `undefined` children

Consider this example:

```
var foo;

if (this.state.bar) foo = <Foo/>;

<TransitionGroup transitionName='baz'>
  {foo}
</TransitionGroup>
```

If `state` is false or undefined, `foo` will be `undefined` and React.addons.TransitionGroup will throw `Uncaught TypeError: Object.keys called on non-object`
