Select None as "Select All"

[stokito]

I have a filter by category and when a user selected none of them then I need it to behave as Select All (e.g. the filter not applied). So I can't use the Select All option but I made instead a manual "Select All" option with just an empty value. It works but now I need to deselect other options when clicking on it. I can't do this inside of the onChange listener because there is no $this.