org.el: org-xor-value return one of values or nil

Signed-off-by: Łukasz Stelmach <stlman@poczta.fm>
ox-taskjuggler.el: try to get id from resource_id too

Signed-off-by: Łukasz Stelmach <stlman@poczta.fm>
ox-taskjuggler.el: assign the id to the projecect before querying it

Signed-off-by: Łukasz Stelmach <stlman@poczta.fm>