[backend/masto-client] Make hashtag history placeholder an array

This should improve compatibility with some mastodon clients.

Iceshrimp.Backend/Controllers/Mastodon/Schemas/Entities/TagEntity.cs

@@ -7,5 +7,6 @@ public class TagEntity
 	[J("name")]      public required string Name      { get; set; }
 	[J("url")]       public required string Url       { get; set; }
 	[J("following")] public required bool   Following { get; set; }
-	[J("history")]   public          object History   => new();
+
+	[J("history")] public object[] History => [];
 }