Allow ClientExtensionResults to be nullable for backwards compatibili…

…ty with Apple. (#627)

src/Service/Fido2Service.cs

@@ -235,7 +235,7 @@ public async Task<TokenResponse> RegisterCompleteAsync(RegistrationCompleteDTO r
             Nickname = request.Nickname,
             BackupState = success.Result.IsBackedUp,
             IsBackupEligible = success.Result.IsBackupEligible,
-            IsDiscoverable = request.Response.Extensions.CredProps?.Rk,
+            IsDiscoverable = request.Response.ClientExtensionResults?.CredProps?.Rk,
         });
 
         var tokenData = new VerifySignInToken