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 
const currentComponentErrorInfo =
    'Functions are not valid as a React child. This may happen if ' +
    'you return a Component instead of <Component /> from render. ' +
    'Or maybe you meant to call this function rather than return it.' +
    (getCurrentFiberStackAddendum() || '');
  if (ownerHasFunctionTypeWarning[currentComponentErrorInfo]) {
    return;
  }
  ownerHasFunctionTypeWarning[currentComponentErrorInfo] = true;
  warning(
    false,
    'Functions are not valid as a React child. This may happen if ' +
      'you return a Component instead of <Component /> from render. ' +
      'Or maybe you meant to call this function rather than return it.%s',
    getCurrentFiberStackAddendum() || '',
  );
}
/
/
/
/
function ChildReconciler(shouldTrackSideEffects) {