Slicer to allow null values

Hi, first post here.  I have a matrix which I am running with some slicers at the top.  Some rows are showing as blanks in my matrix which is correct as not everyone will have an allocation recorded against them.  When I added a slicer at the top to filter to show the number in the matrix, it doesnt allow me to show null values as an option in the slicer

I thought I could get round this and created a new measure to add 0 to replace the null values, but when I do that, the matrix doesnt filter out the 0 values