slapos_subscription_request: constraint: fixup empty price check

master/bt5/slapos_subscription_request/PropertySheetTemplateItem/portal_property_sheets/SlapOSSubscriptionRequestConstraint/empty_price_constraint.xml

@@ -24,7 +24,7 @@
         </item>
         <item>
             <key> <string>expression</string> </key>
-            <value> <string>python: not ((not context.getPrice()) and (context.getSourceSection(\'\') != context.getDestinationSection(\'\')))</string> </value>
+            <value> <string>python: context.getPrice() or (context.getDestinationSection(\'\') in (context.getSourceSection(\'\'), \'\'))</string> </value>
         </item>
         <item>
             <key> <string>id</string> </key>