content search webpart ignore querystring filter when empty.

I'm using a standard content search web part. I have a querystring filter on the HRPortalRole managed property like this:

HRPortalRole:{QueryString.role}

When I provide a value in the querystring, everything is working fine but if I don't specify a value in the querystring it is not showing any content.

I basically want to ignore the filter when it is not there. So if my url doesn't contain the role parameter in the query string I just want to show all content no matter what role it has. Is this somehow possible?