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

[hojberg]

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

[zpao]

Want to take a go at fixing this? I think it would be a good opportunity to learn a bit more about a constrained subset of internals.

[hojberg]

@zpao yea - i'll give it a shot :)