That’s what my partner used to say to me before he got the diagnosis. It sounded as if he never had any thoughts in his head. It put a strain on our relationship too, because he used to answer this those times I noticed he wasn’t listening while I was talking and asked what he was thinking about. Sometimes it seemed like he didn’t care at all.
Turns out he has too many thoughts in his head, and “nothing” was indeed the easiest answer. I guess it doesn’t really matter what the truth is to a random person, but it does matter to your friends and family.
The jury is still out.