Fix system messages always being displayed above standard messages

Closes https://github.com/ppy/osu/issues/12509.

osu.Game/Online/Chat/InfoMessage.cs

@@ -8,10 +8,8 @@ namespace osu.Game.Online.Chat
 {
     public class InfoMessage : LocalMessage
     {
-        private static int infoID = -1;
-
         public InfoMessage(string message)
-            : base(infoID--)
+            : base(null)
         {
             Timestamp = DateTimeOffset.Now;
             Content = message;

osu.Game/Online/Chat/Message.cs

@@ -59,7 +59,7 @@ namespace osu.Game.Online.Chat
             return Id.Value.CompareTo(other.Id.Value);
         }
 
-        public virtual bool Equals(Message other) => Id == other?.Id;
+        public virtual bool Equals(Message other) => Id.HasValue && Id == other?.Id;
 
         // ReSharper disable once ImpureMethodCallOnReadonlyValueField
         public override int GetHashCode() => Id.GetHashCode();