Allow use of `elems` without children

[sandalwoodbox]

Because there is no way to extract translation strings from React, I'm using Localized components without children. For example:

<h1><Localized id="verify-email-page-title" /></h1>

This gets replaced by the translation just fine. However, if I want to use elems I suddenly need to have an empty element inside:

<p>
  <Localized
    id="verify-email-success"
    elems={{ loginLink: <Link to="/login"></Link> }}
  >
    <span></span>
  </Localized>
</p>

It would be great to be able to rely on elems as a signal that I want to use overlays regardless of whether the content inside looks like HTML.

[sandalwoodbox]

It's possible there's a situation where I would want elems but not want to use an overlay, though that seems unlikely?