Warning for leaf routes w/ no element

[mjackson]

We should warn when the leaf route has no element prop. In this case, the element would default to an <Outlet> that renders null, which can be confusing.

let routes = (
  <Routes>
    {/* When the URL is /layout, this route will match but won't render anything! */}
    <Route path="layout">
      <Route path="page" element={...} />
    </Route>
  </Routes>
);

[mjackson]

Fixed in #8150