fix: parsing warnings in root response

[p4veI]

Hello, we've come across a bug in XML parsing when receiving Warnings in the root of the XML response on e.g. Invoices. Supposedly this issue is related to the recent addition of https://developer.xero.com/documentation/api/efficient-data-retrieval checks in Xero's API.

I believe the handling of the warnings was not working properly. The warnings element was parsed as one of the elements which resulted in storing it in the elements array and was then returned to us as a member of the response elements as one of the invoices. But all the element values were null, this caused an error as based on type definitions the model does not allow null on either of the elements of an Invoice model.

I included the improved parsing in the JSON part as well, but I have not tested this behaviour with the API, but believe it will be the same.