Fix potential null referenced in SeedingScreen

Also ensure that any update operations only occur when the seeding
screen is displayed. They were running in the background until now.

osu.Game.Tournament/Models/SeedingBeatmap.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using Newtonsoft.Json;
 using osu.Framework.Bindables;
 
@@ -13,7 +11,7 @@ namespace osu.Game.Tournament.Models
         public int ID;
 
         [JsonProperty("BeatmapInfo")]
-        public TournamentBeatmap Beatmap;
+        public TournamentBeatmap? Beatmap;
 
         public long Score;