https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
  ): null {
    if (!shouldTrackSideEffects) {
      /
      return null;
    }
    /
    /
    let childToDelete = currentFirstChild;
    while (childToDelete !== null) {
      deleteChild(returnFiber, childToDelete);
      childToDelete = childToDelete.sibling;
    }
    return null;
  }
  function mapRemainingChildren(