Appease message pack new inspections

Author: peppy

CodeAnalysis/osu.globalconfig

@@ -51,8 +51,11 @@ dotnet_diagnostic.IDE1006.severity = warning
 # Too many noisy warnings for parsing/formatting numbers
 dotnet_diagnostic.CA1305.severity = none
 
+# messagepack complains about "osu" not being title cased due to reserved words
+dotnet_diagnostic.CS8981.severity = none
+
 # CA1507: Use nameof to express symbol names
-# Flaggs serialization name attributes
+# Flags serialization name attributes
 dotnet_diagnostic.CA1507.severity = suggestion
 
 # CA1806: Do not ignore method results

osu.Game/Online/API/ModSettingsDictionaryFormatter.cs

@@ -10,10 +10,12 @@
 
 namespace osu.Game.Online.API
 {
-    public class ModSettingsDictionaryFormatter : IMessagePackFormatter<Dictionary<string, object>>
+    public class ModSettingsDictionaryFormatter : IMessagePackFormatter<Dictionary<string, object>?>
     {
-        public void Serialize(ref MessagePackWriter writer, Dictionary<string, object> value, MessagePackSerializerOptions options)
+        public void Serialize(ref MessagePackWriter writer, Dictionary<string, object>? value, MessagePackSerializerOptions options)
         {
+            if (value == null) return;
+
             var primitiveFormatter = PrimitiveObjectFormatter.Instance;
 
             writer.WriteArrayHeader(value.Count);

osu.Game/Online/Multiplayer/MatchTypes/TeamVersus/TeamVersusUserState.cs

@@ -5,6 +5,7 @@
 
 namespace osu.Game.Online.Multiplayer.MatchTypes.TeamVersus
 {
+    [MessagePackObject]
     public class TeamVersusUserState : MatchUserState
     {
         [Key(0)]

osu.Game/Users/UserActivity.cs

@@ -54,6 +54,10 @@ public class ChoosingBeatmap : UserActivity
         }
 
         [MessagePackObject]
+        [Union(12, typeof(InSoloGame))]
+        [Union(23, typeof(InMultiplayerGame))]
+        [Union(24, typeof(SpectatingMultiplayerGame))]
+        [Union(31, typeof(InPlaylistGame))]
         public abstract class InGame : UserActivity
         {
             [Key(0)]