org.el: org-xor-value return one of values or nil
ox-taskjuggler.el: try to get id from resource_id  too
[RFC] ox-taskjuggler.el: assign the id to the projecect before querying it

The (list project) seems like a hack but I don't know how to get rid
of org-element-map and work with a single item in
org-taskjuggler-assign-project-ids.
simplyfy project_id assignment