“If all things are empty, or nothing, then there is not anything to be empty.”
In a way, that is the point. All that appears to be, already is nothing.
There are no ‘things’, of any sort, in nothingness. Therefore, there are no things to be—or not to be—nothing, or empty.
So it is not that there are things that are, or are not, separate from nothingness. In nothingness, there are no ‘things’ to be separate from anything. There is not any thing.
You, for example, are not separate from nothingness. Nothingness (or emptiness) being the condition, “you” do not exist.
You, to put it another way, are already empty of reality—in nothingness—now.