Replace Null Values with 0 across Week Dimension

[aplask105]

Hello all - 

I have a Week Dimension table (dim_week_end) and an Activity fact table (fact_activity).

I am attepting to create an average activity calculation over a 4 week period based on a count(distinct) of activity records for each respective employee. 

I've been able to create the following: 

The result suffices, however I can not figure out how to count 0s where there is no Activity record for a given week. My research has led me to try adding '+0' after the DistinctCount() calc, but that compromises my week_end range (If I have 4 weeks filterd, all weeks from the data set will show in the matrix). 

So for the above, the average would be 3.5