When wrapping null nasty errors appear

[kirill-konshin]

Starting from React 15 it is OK to return null from render function.

When this happens lib throws an error:

inview.js:51 Uncaught TypeError: Cannot read property 'getBoundingClientRect' of undefined
    at getBoundingBox (inview.js:51)
    at ReactInview.handleScroll (inview.js:135)
    at invokeFunc (debounce.js:95)
    at trailingEdge (debounce.js:142)
    at timerExpired (debounce.js:130)

So an additional check is required here https://github.com/BausCode/react-inview/blob/master/src/inview.jsx#L101 if element is falsy then abort.