More precisely, the $address string is empty if they're not in your IAL, which could potentially be disabled with /ial off
In addition, the IAL contains addresses only if mirc sees them "do something". So if you enter a channel with 1000 nicks, none of them will be in your ial unless they were already there due to them being in one of your other channels, or until they send a channel message, change their nick, get op/voiced by someone etc.
To add all 1000 people to your ial: /who #channelname
You need to be careful about just having a script doing the /who whenever you join a channel, because if you autojoin 20 large channels and send 20 /who at the same time...
You can also add someone to your ial individually like /who nick or /whois nick
