Filter out empty references and numbers when joining them from sales for UBL invoice

Closes #14652

modules/edocument_ubl/edocument.py

@@ -180,12 +180,14 @@ class Invoice(Model):
     @cached_property
     def sale_reference(self):
         if hasattr(self.invoice, 'sales'):
-            return ','.join(s.reference for s in self.invoice.sales)
+            return ','.join(
+                filter(None, (s.reference for s in self.invoice.sales)))
 
     @cached_property
     def sale_number(self):
         if hasattr(self.invoice, 'sales'):
-            return ','.join(s.number for s in self.invoice.sales)
+            return ','.join(
+                filter(None, (s.number for s in self.invoice.sales)))
 
     @property
     def taxes(self):