Official USCIS instructions say to enter "N/A". Do not give USCIS the impression that you accidentally forgot to answer a question.
https://www.uscis.gov/sites/default/files/document/forms/i-485instr.pdf
Answer all questions fully and accurately. If a question does not apply to you (for example, if you have never been married and the question asks, “Provide the name of your current spouse”), type or print “N/A,” unless otherwise directed. If your answer to a question which requires a numeric response is zero or none (for example, “How many children do you have” or “How many times have you departed the United States”), type or print “None,” unless otherwise directed.