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,
  element: ReactElement,
) {
  let mixedRef = element.ref;
  if (
    mixedRef !== null &&
    typeof mixedRef !== 'function' &&
    typeof mixedRef !== 'object'
  ) {
    if (__DEV__) {
      if (returnFiber.mode & StrictMode) {
        const componentName = getComponentName(returnFiber) || 'Component';
        if (!didWarnAboutStringRefInStrictMode[componentName]) {
          warning(
            false,
            'A string ref, "%s", has been found within a strict mode tree. ' +
              'String refs are a source of potential bugs and should be avoided. ' +
              'We recommend using createRef() instead.' +
              '\n%s' +
              '\n\nLearn more about using refs safely here:' +
              '\nhttps://fb.me/react-strict-mode-string-ref',
            mixedRef,
            getStackAddendumByWorkInProgressFiber(returnFiber),
          );
          didWarnAboutStringRefInStrictMode[componentName] = true;