Return null for unknown type in getStateData to prevent empty URL request

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/stores/compare.ts

@@ -55,6 +55,8 @@ export const useCompareStore = defineStore("compareStore", {
         case "filter":
           api = `/api/filters/${id}/discover`;
           break;
+        default:
+          return null;
       }
       try {
         const response = await apiClient.get(api);