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 
case REACT_PORTAL_TYPE: {
          const matchedFiber =
            existingChildren.get(
              newChild.key === null ? newIdx : newChild.key,
            ) || null;
          return updatePortal(
            returnFiber,
            matchedFiber,
            newChild,
            expirationTime,
          );
        }
      }
      if (isArray(newChild) || getIteratorFn(newChild)) {
        const matchedFiber = existingChildren.get(newIdx) || null;
        return updateFragment(
          returnFiber,
          matchedFiber,
          newChild,
          expirationTime,
          null,
        );
      }
      throwOnInvalidObjectType(returnFiber, newChild);
    }
    if (__DEV__) {
      if (typeof newChild === 'function') {
        warnOnFunctionType();
      }
    }