Always write [HitObjects] to file

I think this is expected. If not, there's an alternative solution.

osu.Game/Beatmaps/Formats/LegacyBeatmapEncoder.cs

@@ -235,11 +235,11 @@ namespace osu.Game.Beatmaps.Formats
 
         private void handleHitObjects(TextWriter writer)
         {
+            writer.WriteLine("[HitObjects]");
+
             if (beatmap.HitObjects.Count == 0)
                 return;
 
-            writer.WriteLine("[HitObjects]");
-
             foreach (var h in beatmap.HitObjects)
                 handleHitObject(writer, h);
         }