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 
</svg>
                <image {...expectSVG} />
              </svg>,
            )}
            <image {...expectSVG} />
            <foreignObject {...expectSVG}>
              <p {...expectHTML} />
              {usePortal(<p {...expectHTML} />)}
              <p {...expectHTML} />
            </foreignObject>
          </svg>
          <image {...expectSVG} />
        </svg>
        <p {...expectHTML} />
      </div>,
    );
    assertNamespacesMatch(
      <div {...expectHTML}>
        <svg {...expectSVG}>
          <foreignObject {...expectSVG}>
            <p {...expectHTML} />
            {usePortal(
              <svg {...expectSVG}>
                <image {...expectSVG} />
                <svg {...expectSVG}>
                  <image {...expectSVG} />
                  <foreignObject {...expectSVG}>
                    <p {...expectHTML} />
                  </foreignObject>
                  {usePortal(<p {...expectHTML} />)}