session: fix crash when storing view-source URI without host component

Fixes, for example:

epiphany-CRITICAL **: 10:29:44.752: Refusing to save session due to invalid URL view-source:file:///home/mcatanzaro/a.html

I'm not sure it's actually a good idea to store view-source URIs in the
session state, but let's just make a small change to handle it
gracefully instead of rethinking that.

Fixes #1987

Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/1300>