glibproxyresolver: validate proxy URIs
glibproxyresolver: validate proxy URIs

This adds an extra layer of safety to ensure our GProxyResolver only
ever returns URIs that GLib considers valid. Otherwise, if libproxy
returns something silly like an empty string, nothing prevents us from
passing it along.

Part-of: <https://gitlab.gnome.org/GNOME/glib-networking/-/merge_requests/212>

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking