Resolving unamed url raised Resolver404 and prevent preview

[weber-s]

Hello!

In our setup, the resolver in the middleware couldn't resolve correclty the url "/":

ipdb> request.path_info
'/'
ipdb> request
<WSGIRequest: GET '/'>

And so the resolve(request.path_info) lead to Resolver404, that breaks the middleware and the preview was always redirected to the home.

This PR fix it.

However, I did not understand why the middleware is having this request of path "/" , since I am in a preview.