The easiest way programatically would be to simply add a default value to the message text area.
<textarea name="message" cols="35" rows="8" wrap="soft" style="width%"><?=$info?></textarea></td>
would become something like:
<textarea name="message" cols="35" rows="8" wrap="soft" style="width%"><?php
if(!empty($info)) {
echo $info;
}
else {
echo "message";
}
?></textarea></td>
The alternative would be to edit /include/class.ticket.php to make message not required. This is also easy to do. See line: 1850
$fields = array('type'=>'text', 'required'=>1, 'error'=>'Message required');
would become:
$fields = array('type'=>'text', 'required'=>0, 'error'=>'Message required');
Making either change would essentially mean no one would ever have to enter a ticket body message ever, and I don't recommend it.