When I read I thought of this.
This is honestly a guy thing that you are dealing with. I know a lot of guys that when we do talk about it we say the same thing, when we are thinking nothing, we're really thinking nothing, or something that we honestly don't think you have any interest in. In my case when my girlfriend tries to ask what I'm thinking, it turns out I'm either thinking of rap lyrics to a song I want to do at karoke, or "left foot right foot, turn at the car over there, left foot right foot."
Don't worry about it.