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 
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {ReactElement} from 'shared/ReactElementType';
import type {ReactPortal} from 'shared/ReactTypes';
import type {Fiber} from 'react-reconciler/src/ReactFiber';
import type {ExpirationTime} from 'react-reconciler/src/ReactFiberExpirationTime';
import getComponentName from 'shared/getComponentName';
import {Placement, Deletion} from 'shared/ReactTypeOfSideEffect';
import {
  getIteratorFn,
  REACT_ELEMENT_TYPE,
  REACT_FRAGMENT_TYPE,
  REACT_PORTAL_TYPE,
} from 'shared/ReactSymbols';
import {
  FunctionalComponent,
  ClassComponent,
  HostText,
  HostPortal,
  Fragment,
} from 'shared/ReactTypeOfWork';
import {getStackAddendumByWorkInProgressFiber} from 'shared/ReactFiberComponentTreeHook';
import emptyObject from 'fbjs/lib/emptyObject';