Fix system user attempting to show in profile overlay

osu.Game/Users/User.cs

@@ -144,5 +144,14 @@ namespace osu.Game.Users
         public Badge[] Badges;
 
         public override string ToString() => Username;
+
+        /// <summary>
+        /// A user instance for displaying locally created system messages.
+        /// </summary>
+        public static User SystemUser { get; } = new User
+        {
+            Username = "system",
+            Id = 0
+        };
     }
 }