When no classes are selected, classNames should return null

ClassNames with no classes selected return empty string (''). For using it in React with jsx, it would be much better to return null. Example:

<p className={classNames({ first: false, second: false)>
Hello World!
</p>

This results in <p class>Hello World!</p>, while <p>Hello World!</p> would be correct. The second would happen if classNames returned null in such case.