always show the parent id in the payload

- note that the parentId is just an intermediate solution (I fear for a long time)
-> better use nice references ("parent": { "href": "foooo" })
-> alternatively go the relations route...
- that is also why I refuse to list it in the schema... An id just does not make sense...

lib/api/v3/work_packages/form/work_package_payload_representer.rb

@@ -101,7 +101,10 @@ module API
                    },
                    setter: -> (value, *) { represented.description = value['raw'] },
                    render_nil: true
-          property :parent_id, writeable: true
+
+          property :parent_id,
+                   writeable: true,
+                   render_nil: true
 
           property :project_id,
                    getter: -> (*) { nil },