Enable nullabe reference types for MediaBrowser.Model

Emby.Server.Implementations/Cryptography/CryptographyProvider.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Security.Cryptography;
@@ -134,8 +136,6 @@ namespace Emby.Server.Implementations.Cryptography
                 _randomNumberGenerator.Dispose();
             }
 
-            _randomNumberGenerator = null;
-
             _disposed = true;
         }
     }