Something that was “nullified” effectively never happened. So I would say your biological father and your Mom “were never married”.
However, since your Mom and her former husband supposedly were indeed still married at the time you were born - your “parents at time of birth” were married (and her former husband was entered as your legal parent?) - then their current legal state would be “divorced.”
You probably should verify that with someone practicing family law - and maybe also obtain registry records so that you have factual information going forward.