fix(fastfetch): handle null omnixy.preset

`${cfg.preset or "custom"}` only falls back when the attr is missing,
not when it's null — and omnixy.preset defaults to null. A host that
doesn't set a preset (any consumer of nixosModules.omnixy) hit 'cannot
coerce null to a string'. Use an explicit null check. Surfaced by the
dependency-model consumer test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/fastfetch.nix

@@ -165,7 +165,7 @@ in
           },
           {
             "type": "custom",
-            "format": "  󰣇 Preset: ${cfg.preset or "custom"}",
+            "format": "  󰣇 Preset: ${if cfg.preset == null then "custom" else cfg.preset}",
             "keyColor": "magenta"
           },
           {