the null values that you are getting are caused by a mismatch of the JOIN keys during the joining process. Within the JOIN load, these values can't be tested for NULL yet. Technically speaking, they are not NULL, they are MISSING.
In order to overcome this issue, you need to reload your resulting table one more time, using a RESIDENT load, and replace nulls with the desired values. You can also use more elegant NULL handling techniques such as NullAsValue. I describe null handling techniques, along with many other advanced scripting techniques, in my new book "QlikView Your Business".
Thanks everyone for your help.
I found, also, another solution for my issue using the "Exists" and Not Exists" function.
I am attaching the file where in the CONCATENATE TAB you can look on my solution.
What do you think?