dunst: only use -config flag when configFile is set

[Anomalocaridid]

Description

This ensures that in dunst's systemd service, the -config flag is only used when configFile is set. This is because when passing -config to dunst, dunst does not look for config files other than the specified path. This breaks dunst's drop-in functionality, where files can be placed in ~/.config/dunst/dunstrc.d to override settings in ~/.config/dunst/dunstrc.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

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Maintainer CC

@rycee

[rycee]

Thanks for the contribution!

[Anomalocaridid]

It was my pleasure!