remote-add: Don't make the repo existence optional
remote-add: Don't make the repo existence optional

A repository needs to exist for a remote to be written inside,
otherwise the OstreeRepo instance will be NULL and a crash will
occur once the instance is accessed.

Fixes #3612.

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak