Return "NotAvailable" when no UserChannel data is present. (#1569)

* Return "NotAvailable" when no UserChannel data is present.

* Return ObjectInvalid for undefined parameter kinds.

* No need to specify which, there's only one.

* Just works as a literal string.

Ryujinx.HLE/HOS/UserChannelPersistence.cs

@@ -31,7 +31,9 @@ namespace Ryujinx.HLE.HOS
 
         public byte[] Pop()
         {
-            return _userChannelStorages.Pop();
+            _userChannelStorages.TryPop(out byte[] result);
+
+            return result;
         }
 
         public bool IsEmpty => _userChannelStorages.Count == 0;