[fixed] Ignore falsy routes

[tgriesser]

I just tried to do something like this:

<Route name="app" path="/" handler={Application}>
      <Route name="job" path="job/:job_id/?" handler={JobLayout}>
          <Route name="report" handler={Report} />
          {__DEBUG__ && <Route name="debug" handler={Debug} />}
          <DefaultRoute handler={JobForm} />
     </Route>
</Route>

Where the __DEBUG__ var is injected via webpack at compile time. This works fine during development, but in production, the line becomes equivalent to {undefined}. This pattern for conditionally rendering nodes is fairly common in React. I thought it might make sense to allow this here by skipping over falsy values in the createRoutesFromChildren function.

[mjackson]

Makes sense. Thanks for the PR @tgriesser !