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 
current: Fiber | null,
  workInProgress: Fiber,
): void {
  invariant(
    current === null || workInProgress.child === current.child,
    'Resuming work not yet implemented.',
  );
  if (workInProgress.child === null) {
    return;
  }
  let currentChild = workInProgress.child;
  let newChild = createWorkInProgress(
    currentChild,
    currentChild.pendingProps,
    currentChild.expirationTime,
  );
  workInProgress.child = newChild;
  newChild.return = workInProgress;
  while (currentChild.sibling !== null) {
    currentChild = currentChild.sibling;
    newChild = newChild.sibling = createWorkInProgress(
      currentChild,
      currentChild.pendingProps,
      currentChild.expirationTime,
    );
    newChild.return = workInProgress;
  }
  newChild.sibling = null;
}