render nothing if the param is the number zero

In react, the number zero will be rendered directly, rather than taken as false or null.

`let items = ['one', 'two', 'three'];
document.body.innerHTML = (
	<div class="foo">
		<h1>Hi!</h1>
		<p>Here is a list of {items.length} items:</p>
		<ul>
			{ items.map( item => (
				<li>{ 1 }</li>
			)) }
		</ul>
	</div>
);
`