Don’t render an ID if wrapper_html[:id] is nil

At the moment, if `wrapper_html[:id]` is `nil`, the default wrapper ID gets rendered. It's sometimes desirable to take the `id` attribute off entirely—for example, in a page where client-side JavaScript may make an indeterminate number of copies of a form—but there hasn't been a good way to do that.

This pull request changes the behavior of `wrapper_html` so that if `:id` is `nil`, no `id` attribute gets rendered on the wrapper.