Yeah, I've ran into that before and I can't remember what I did, but I would start with checking Include NULLs in the report options. Next would be working the repository to force the behavior by leveraging the total levels of the dimensional hierarchy (I would avoid this as it would be very messy looking).
Thanks for the reply.
Including the NULLs will result in adding all the irrelevant values from the second table, so it will not work.
Didn't quite get the second part of your answer.