Fix condition to show encryption warning

[salim-b]

Although the official Flatpak documentation about XDG base directories states that

These environment variables are always set by flatpak and override any host values.

XDG_CONFIG_HOME can be unset, apparently?

This fixes #751 (hopefully).

Please note that I don't have much experience with Flatpak internals, so please thoroughly review this change.

[flathubbot]

Started test build 150298

[flathubbot]

Build 150298 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/133389/org.signal.Signal.flatpakref

[salim-b]

Ok, I just realized I did a beginner mistake with

> flatpak run --command=echo org.signal.Signal "${XDG_CONFIG_HOME}/Signal"
/Signal

to test whether the env var is set for me. 🙈

Correct command:

> flatpak run --command=sh org.signal.Signal -c 'echo "${XDG_CONFIG_HOME}/Signal"'
/var/home/salim/.var/app/org.signal.Signal/config/Signal

So I dunno whether this PR still makes any sense.

[bbhtt]

Yea XDG_CONFIG_HOME can never be unset inside the sandbox. It's how Flatpak redirects it to ~/.var/app/$app-id/config instead of ~/.config