fix(otp): Handle case when last slot is deleted

Fixes #182

Author: DennisDyallo

Yubico.YubiKey/src/Yubico/YubiKey/Pipelines/OtpErrorTransform.cs

@@ -62,16 +62,20 @@ public ResponseApdu Invoke(CommandApdu command, Type commandType, Type responseT
 
                 // If we see the sequence number change, we can assume that the configuration was applied successfully. Otherwise
                 // we just invent an error in the response.
-                return afterSequence != expectedSequence
-                    ? new ResponseApdu(responseApdu.Data.ToArray(), SWConstants.WarningNvmUnchanged)
-                    : responseApdu;
+                // Note that when deleting the "last" slot so no slots are valid, the programming sequence
+                // is set to 0, which is also valid.
+                return afterSequence == expectedSequence || afterSequence == 0
+                    ? responseApdu
+                    : FailedApdu(responseApdu.Data.ToArray());
             }
             catch (KeyboardConnectionException e)
             {
                 _logger.LogWarning(e, "Handling keyboard connection exception. Translating to APDU response.");
 
-                return new ResponseApdu(Array.Empty<byte>(), SWConstants.WarningNvmUnchanged);
+                return FailedApdu([]);
             }
+
+            static ResponseApdu FailedApdu(byte[] data) => new(data, SWConstants.WarningNvmUnchanged);
         }
 
         public void Setup() => _nextTransform.Setup();