This is not nothingness. Dreamless sleep is still having bhavaṅga mind.
Ok, fine, below is me addressing the issue from AN4.173.
https://www.reddit.com/r/Buddhism/comments/1awbf40/misconception_theres_something_after_parinibbāna/?share_id=7VazGqBTjobXZruycY5dN&utm_content=1&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=1
I think it’s a nothingness which is so nothing, that to say nothing to it is to add a concept to it and it’s an insult to it. So if anything, it’s beyond the concept of nothing to even further nothing. Level 9 nothingness. To be fair, to describe level 9 nothingness still uses concept, but level 8 throws out concepts already. @Dhabba seeing the youtube video of 9 levels of nothingness would help you to get a better sense of why dreamless sleep is still full of somethings.