Fix error in test preventing Moq update (#10096)

tests/Jellyfin.Api.Tests/Auth/DefaultAuthorizationPolicy/DefaultAuthorizationHandlerTests.cs

@@ -66,7 +66,7 @@ namespace Jellyfin.Api.Tests.Auth.DefaultAuthorizationPolicy
 
             _userManagerMock
                 .Setup(u => u.GetUserById(It.IsAny<Guid>()))
-                .Returns<User>(null);
+                .Returns<User?>(null);
 
             var claims = new[]
             {