undefined value in connect function causes silent failure

I had a bug where my `connect` function was returning something like `{"propName": undefined}`. When that happened, React just silently failed to render the whole component tree. There were no errors in the console, it just acted like I never called `ReactDOM.render()` on the top-level component.

Would it be possible to show an error in that circumstance? I'm not sure if this is a problem with your library or with React, but it took a while to hunt down the issue. I'm on React 15.1.0 and Chrome 51 if that helps.