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 
return null;
        }
        return updateFragment(
          returnFiber,
          oldFiber,
          newChild,
          expirationTime,
          null,
        );
      }
      throwOnInvalidObjectType(returnFiber, newChild);
    }
    if (__DEV__) {
      if (typeof newChild === 'function') {
        warnOnFunctionType();
      }
    }
    return null;
  }
  function updateFromMap(
    existingChildren: Map<string | number, Fiber>,
    returnFiber: Fiber,
    newIdx: number,
    newChild: any,
    expirationTime: ExpirationTime,
  ): Fiber | null {
    if (typeof newChild === 'string' || typeof newChild === 'number') {
      /
      /
      const matchedFiber = existingChildren.get(newIdx) || null;