#1: Interstellar space is far from empty, completely correct. Let's think of a little bit of vacuum in the lab somewhere, in an isolated place. Minimal radiation from outside, virtually no particles inside. Then we have a close approximation of an ideal vacuum. This is the closest we can get to material nothingness. Of course, we can say that there is still something material in there, but then, material nothingness does not exist in our universe, making the word rather useless. So let's relax the criteria a bit, and say that vacuum, despite the fluctuations, is nothingness. I am not quite certain about the following, but I suspect that quantum mechanics themselves forbid a perfect nothingness, that is, vacuum without the fluctuations.
#2: The Null and value of a variable thingie. What I was trying to express is that "space" is not matter, thus it's not quite right to say that vacuum contains space, thus something. Space is a way of adressing something, like an array index. If we have "int a;", and "a = NULL;", that is almost the same. We have nothingsness, i.e. the concept of NULL, in place no. 3.
To conclude, I just meant to say that to me, nothingness is most strongly associated with vacuum, which contrary to common expectations, fluctuates. NULL is nothingness as well, but in a different way. Sorry for messing this up so much.