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 
}
    if (!child._store || child._store.validated || child.key != null) {
      return;
    }
    invariant(
      typeof child._store === 'object',
      'React Component in warnForMissingKey should have a _store. ' +
        'This error is likely caused by a bug in React. Please file an issue.',
    );
    child._store.validated = true;
    const currentComponentErrorInfo =
      'Each child in an array or iterator should have a unique ' +
      '"key" prop. See https://fb.me/react-warning-keys for ' +
      'more information.' +
      (getCurrentFiberStackAddendum() || '');
    if (ownerHasKeyUseWarning[currentComponentErrorInfo]) {
      return;
    }
    ownerHasKeyUseWarning[currentComponentErrorInfo] = true;
    warning(
      false,
      'Each child in an array or iterator should have a unique ' +
        '"key" prop. See https://fb.me/react-warning-keys for ' +
        'more information.%s',
      getCurrentFiberStackAddendum(),
    );
  };
}
const isArray = Array.isArray;
function coerceRef(