Fix: public_file_url on customer invoices is now null until the PDF is ready

public_file_url on Company API v2 customer invoices is now null until the invoice PDF has been generated.

Until now, responses always included a URL, even when the PDF was still being generated. Opening that URL early could return a provisional PDF with missing product lines, while the same invoice looked complete in Pennylane.

What changes

POST /customer_invoices returns public_file_url: null while the PDF is being generated.
The same applies to create_from_quote, finalize, update, link_credit_note, and to GET and list calls made before the PDF exists. Once the PDF is ready, public_file_url contains the URL.

What you need to do

If you download the PDF straight after creating an invoice, handle null.
Fetch the invoice again GET /customer_invoices/{id} ) until public_file_url is set.

No schema change: public_file_url was already documented as nullable. Quotes, commercial documents, supplier invoices and the v1 API are not affected.