Return empty string instead of undefined from getFileName on not-found

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

src/stores/compare.ts

@@ -78,6 +78,7 @@ export const useCompareStore = defineStore("compareStore", {
       } catch (error) {
         apiError(error, "Failed to load the Compare's file name.");
       }
+      return "";
     },
   },
 });