bt5_config: constructTemplatePath should return None in case the path doesn't exist yet. Needed for cases where we are creating a new path_item object

bt5_config: Override accessors for type_workflow_list accroding to need for portal_workflow chain bootstrap

bt5_config: Add _delPropValue function to Base so not to use the similar function from OFS.ObjectManager which is more for attributes deletion rather than property deletion