fix: set custom invoice number only if available (#228) (#231)

The custom invoice number might not be generated until submission.
For einvoice validation to pass during previous draft saves, we need to
fall back to doc.name as long as the custom number is not available.

(cherry picked from commit 7f79976)

Co-authored-by: Raffael Meyer <14891507+barredterra@users.noreply.github.com>

Author: mergify[bot]

eu_einvoice/european_e_invoice/custom/sales_invoice.py

@@ -213,12 +213,11 @@ def _set_header(self):
 		sales_invoice_number_field = frappe.db.get_single_value(
 			"E Invoice Settings", "sales_invoice_number_field"
 		)
-		if sales_invoice_number_field:
-			invoice_name = self.invoice.get(sales_invoice_number_field)
-		else:
-			invoice_name = self.invoice.name
 
-		self.doc.header.id = invoice_name
+		if sales_invoice_number_field and (invoice_number := self.invoice.get(sales_invoice_number_field)):
+			self.doc.header.id = invoice_number
+		else:
+			self.doc.header.id = self.invoice.name
 
 		# https://unece.org/fileadmin/DAM/trade/untdid/d16b/tred/tred1001.htm
 		if self.invoice.is_return: