Allow nullable host in DesktopStorage ctor

The base `NativeStorage` is null-tolerant anyways.

This was relied on in a game-side usage:

	511451770c/osu.Game.Tournament/IPC/FileBasedIPC.cs (L67)

osu.Framework/Platform/DesktopStorage.cs

@@ -5,7 +5,7 @@ namespace osu.Framework.Platform
 {
     public class DesktopStorage : NativeStorage
     {
-        public DesktopStorage(string path, DesktopGameHost host)
+        public DesktopStorage(string path, DesktopGameHost? host)
             : base(path, host)
         {
         }