Add changes according to review

+ Make `ApplicationId` nullable
+ Move ctor after props

src/Discord.Net.Core/Entities/Activities/Game.cs

@@ -5,11 +5,10 @@ namespace Discord
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class Game : IActivity
     {
-        internal Game() { }
-
         public string Name { get; internal set; }
         public ActivityType Type { get; internal set; }
 
+        internal Game() { }
         public Game(string name, ActivityType type = ActivityType.Playing)
         {
             Name = name;

src/Discord.Net.Core/Entities/Activities/GameAsset.cs

@@ -1,15 +1,15 @@
-namespace Discord
+namespace Discord
 {
     public class GameAsset
     {
         internal GameAsset() { }
 
-        internal ulong ApplicationId { get; set; }
+        internal ulong? ApplicationId { get; set; }
         
         public string Text { get; internal set; }
         public string ImageId { get; internal set; }
         
         public string GetImageUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
-            => CDN.GetRichAssetUrl(ApplicationId, ImageId, size, format);
+            => ApplicationId.HasValue ? CDN.GetRichAssetUrl(ApplicationId.Value, ImageId, size, format) : null;
     }
-}
+}

src/Discord.Net.Core/Entities/Activities/SpotifyGame.cs

@@ -7,8 +7,6 @@ namespace Discord
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class SpotifyGame : Game
     {
-        internal SpotifyGame() { }
-
         public string[] Artists { get; internal set; }
         public string AlbumArt { get; internal set; }
         public string AlbumTitle { get; internal set; }
@@ -17,6 +15,8 @@ namespace Discord
         public string SessionId { get; internal set; }
         public TimeSpan? Duration { get; internal set; }
 
+        internal SpotifyGame() { }
+
         public override string ToString() => Name;
         private string DebuggerDisplay => $"{Name} (Spotify)";
     }

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -55,7 +55,7 @@ namespace Discord.WebSocket
         }
 
         // (Small, Large)
-        public static GameAsset[] ToEntity(this API.GameAssets model, ulong appId = 0)
+        public static GameAsset[] ToEntity(this API.GameAssets model, ulong? appId = null)
         {
             return new GameAsset[]
             {