Aggregation with count that includes empty field values
I've not found a way to do this thing, but if any other needs something similar, i end up using Computed Field to handle this case, so i can store the actual field value normally, or another text (in my case 'Not filled').  

Using this computed field instead of the actual field makes everything works...  

But i'm open to change this thing if there's a better way to do this, as this sound a bit hacky...