There is an A# on your naturalization certificate. Since it is available, it is generally recommended to fill in the information, although it is not an error to leave it blank either.
In any event, the I-130 petition if filed correctly will unequivocally label you as a U.S. citizen irrespective of the completion of that field on the G325A.
ATTN: ATTORNEYS: "YOU ARE AT YOUR VOTING LIMIT FOR THE CALENDAR MONTH" is the popup message from AVVO when pressing the "I agree" button for colleagues' answers since the evening of January 22. Please excuse this temporary breach of etiquette until Avvo gets it sorted out.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter, nor should it be viewed as establishing an attorney client relationship of any kind.