2 issues during my implementation of XRechnung writer

[akvamassa]

@landrix hello and thank you so much for this library! I used it to generate Zugferd 2.3 (XRechnung profile) XML.
And I faced to several errors (I think so), after fixing them I was able to finish my task.
I wanna share them with you, so maybe you will check and fix them.

1. In some reason these lines are commented in AddTradeLineItem() function. I have uncommented them for my usage.
   newItem.GrossUnitPrice := grossUnitPrice;
   newItem.NetUnitPrice := netUnitPrice;

2. In TZUGFeRDInvoiceDescriptor23CIIWriter.Save() where we save BuyerOrderReferencedDocument, we write empty element for XRechnung (nothing inside), which lead to warning in validation https://ecosio.com/de/peppol-und-xml-dokumente-online-validieren/ (Element should not be empty). So I removed saving BuyerOrderReferencedDocument for profile XRechnung.