window-manage-views: don't call methods on a NULL NautilusFile

In got_file_info_for_view_selection_callback(), we unconditionally try
to fetch information for the parent directory in case the selection is a
regular file, but we should avoid doing that when the file has no
parent, such as when it's the root of a web server.

https://bugzilla.gnome.org/show_bug.cgi?id=675259