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 
// 
          deleteChild(returnFiber, oldFiber);
        }
      }
      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
      if (previousNewFiber === null) {
        // 
        resultingFirstChild = newFiber;
      } else {
        // 
        // 
        // 
        // 
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
      oldFiber = nextOldFiber;
    }
    if (step.done) {
      // 
      deleteRemainingChildren(returnFiber, oldFiber);
      return resultingFirstChild;
    }
    if (oldFiber === null) {
      // 
      // 
      for (; !step.done; newIdx++, step = newChildren.next()) {
        const newFiber = createChild(returnFiber, step.value, expirationTime);
        if (newFiber === null) {
          continue;