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 
fiber: Fiber,
    pendingProps: mixed,
    expirationTime: ExpirationTime,
  ): Fiber {
    /
    /
    const clone = createWorkInProgress(fiber, pendingProps, expirationTime);
    clone.index = 0;
    clone.sibling = null;
    return clone;
  }
  function placeChild(