I was able to create the base view from this exercise and pass NULL as a parameter successfully for the ‘bv_product_categories’ base view. So, I do not think that passing NULL as a parameter is the reason you encountered this error.
I would recommend you check the following if you are still getting that error:
1) Check that you have the most recent updated version of the Denodo Platform available to you.
2) Check the vdp.log located here: '<DENODO_HOME>/logs/vdp/' to get more details about the error in Virtual DataPort server so that the wsse header error can be pinpointed.
My personal thought is that the error could possibly be related to the internal URL in the SOAP address not responding, or the NULL token not getting passed as a parameter in sufficient time.
Hope this helps!