Enhance Info model to handle optional JSON fields gracefully

This commit updates the Info class to provide default values for optional or missing fields in the JSON payload. This ensures that the model can be successfully instantiated even when some JSON fields are absent or set to null.

frontend/lib/models/skill_tree/info.dart

@@ -11,9 +11,9 @@ class Info {
 
   factory Info.fromJson(Map<String, dynamic> json) {
     return Info(
-      difficulty: json['difficulty'],
-      description: json['description'],
-      sideEffects: List<String>.from(json['side_effects']),
+      difficulty: json['difficulty'] ?? "",
+      description: json['description'] ?? "",
+      sideEffects: List<String>.from(json['side_effects'] ?? []),
     );
   }
 }