slapos_accounting: contraint: source_section is not required on Open Sale Order

Needed for accounting less Open Order.

master/bt5/slapos_accounting/PropertySheetTemplateItem/portal_property_sheets/SlapOSAccountingOpenSaleOrderConstraint/source_section_constraint.xml

@@ -46,7 +46,7 @@
         </item>
         <item>
             <key> <string>min_arity</string> </key>
-            <value> <int>1</int> </value>
+            <value> <int>0</int> </value>
         </item>
         <item>
             <key> <string>portal_type</string> </key>