genvironmentproxyresolver: validate environment variable values

Instead of blindly trusting the environment, let's make sure we have
actually received proper URLs. An empty string will be assumed to mean
"no proxy, please." Any empty environment variable will still cause all
GNOME proxy settings to be ignored, because you might want to use an
empty environment variable to temporarily suppress use of GNOME proxy
settings without messing with XDG_CURRENT_DESKTOP.

Helps with glib#2597

Fixes #189

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