Same null-vs-missing bug as the fastfetch fix, across lib.nix (env var +
status echo), menus.nix, scripts.nix, and fastfetch's about screen:
`cfg.preset or "…"` returns null (not the fallback) because preset
defaults to null. Any consumer that doesn't set a preset hit 'cannot
coerce null to a string'. Replace with explicit null checks.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
