If no matching ad tags for a given tag_id are found, don't emit any ad code into the page at all.

Otherwise undesirable things like this can happen:

<div id='acm-ad-tag-%tag_id%' style='width:%width%px; height:%height%px;'>
	<script type='text/javascript'>
		googletag.cmd.push(function() { googletag.display('acm-ad-tag-%tag_id%'); });
	</script>
</div>