Really only one way to achieve this.
Go into the theme's html templates from the customization area in the ACP, search for ipsDataItem_meta ipsContained_container ipsMessageRow it'll be in the messagelistRow
<div class='ipsDataItem_meta ipsContained_container ipsMessageRow' data-ipsTruncate data-ipsTruncate-type="remove" data-ipsTruncate-size="1 lines">
{{if $row['last_message']}}<span class='ipsType_break ipsContained'>{$row['last_message']->truncated( TRUE )|raw}</span>{{endif}}
</div>
Delete the whole if/endif statement, save. Voila.