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 
return created;
    } else {
      /
      const existing = useFiber(current, portal.children || [], expirationTime);
      existing.return = returnFiber;
      return existing;
    }
  }
  function updateFragment(
    returnFiber: Fiber,
    current: Fiber | null,
    fragment: Iterable<*>,
    expirationTime: ExpirationTime,
    key: null | string,
  ): Fiber {
    if (current === null || current.tag !== Fragment) {
      /
      const created = createFiberFromFragment(
        fragment,
        returnFiber.mode,
        expirationTime,
        key,
      );
      created.return = returnFiber;
      return created;
    } else {
      /
      const existing = useFiber(current, fragment, expirationTime);
      existing.return = returnFiber;
      return existing;