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,
    current: Fiber | null,
    portal: ReactPortal,
    expirationTime: ExpirationTime,
  ): Fiber {
    if (
      current === null ||
      current.tag !== HostPortal ||
      current.stateNode.containerInfo !== portal.containerInfo ||
      current.stateNode.implementation !== portal.implementation
    ) {
      /
      const created = createFiberFromPortal(
        portal,
        returnFiber.mode,
        expirationTime,
      );
      created.return = returnFiber;