Views integration: 'Link field' not available as Views filter

I need to apply a Views filter on a link field, to check whether the field 'is not NULL' (when there is no data in the link field, the Views query will still return NULL, Views gets a result back and the View is not empty, which I want it to be).

When I try to apply this filter though, I can only add 'Content: (field_link) - delta', not the link field itself. Is there a reason for the link field not being available? Or could you add this to the Views integration?