crash on get_absolute_media_url

[Chadys]

Hello,
get_absolute_media_url is used with utils.get_absolute_media_url(self.get_site()), but sometimes get_sitereturns None and so the website crash.
It happen when the page is not routable (not sure yet when that is, in my case it seems to be a complex issue around the usage of wagtail-localize).
I think get_absolute_media_url should check for None values (I can make the PR)