The forum software I run is old. As such, it does not include an option during registration to opt in from admin emails - you have to do that separately. In the process of sending out a targeted email, I notice that the forums are sending it to only 100 of a possible 1200 users who could be in the group. I check the database to see if I can figure out why the number is that low, and it turns out that the other 1100 have never set ANY opt-in option in their profile and so their field is null.

Is it ethically or even legally sound to send the email to the targeted users who have given no opinion on whether they want email or not?

Ahh. Good question. I personally would not bulk-email any group of people who have not specifically asked to receive the email(s). I know I wouldn't like it. But that's just me ;)