made icon optional and default to empty string & adjust all game instances

lib/data/dto/game.dart

@@ -1,6 +1,6 @@
 import 'package:clock/clock.dart';
-import 'package:uuid/uuid.dart';
 import 'package:tallee/core/enums.dart';
+import 'package:uuid/uuid.dart';
 
 class Game {
   final String id;
@@ -18,10 +18,11 @@ class Game {
     required this.ruleset,
     String? description,
     required this.color,
-    required this.icon,
+    String? icon,
   }) : id = id ?? const Uuid().v4(),
        createdAt = createdAt ?? clock.now(),
-       description = description ?? '';
+       description = description ?? '',
+       icon = icon ?? '';
 
   @override
   String toString() {
@@ -49,4 +50,3 @@ class Game {
     'icon': icon,
   };
 }
-