Fix empty field OrderReference/ID

The spec is no longer alloowing empty elements.

Specifying a sales_order_reference but _not_ a purchase_order_reference
lead to having an empty OrderReference/ID field.

lib/xrechnung.rb

@@ -314,7 +314,9 @@ def to_xml(indent: 2, target: "")
         unless self.class.members[:purchase_order_reference].optional && purchase_order_reference.nil? &&
                self.class.members[:sales_order_reference].optional && sales_order_reference.nil?
           xml.cac :OrderReference do
-            xml.cbc :ID, purchase_order_reference
+            unless self.class.members[:purchase_order_reference].optional && purchase_order_reference.nil?
+              xml.cbc :ID, purchase_order_reference
+            end
             unless self.class.members[:sales_order_reference].optional && sales_order_reference.nil?
               xml.cbc :SalesOrderID, sales_order_reference
             end