Veri gelirse görünür, yoksa görümez.

$altmenu=$data->alt_menu1;
if(!empty($altmenu)){
echo "<ul class='dropdown-menu' role='menu'>
        <li><a href='index-2.html'><?php echo $data->alt_menu1; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu2; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu3; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu4; ?></a></li>
        </ul>";
}
ya da

$altmenu=$data->alt_menu1;

$ul_style = '';

if(empty($altmenu)){
$ul_style = 'sytle="display:none"';
}

echo "<ul ".$ul_style." class='dropdown-menu' role='menu'>
        <li><a href='index-2.html'><?php echo $data->alt_menu1; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu2; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu3; ?></a></li>
        <li><a href='index-2.html'><?php echo $data->alt_menu4; ?></a></li>
        </ul>";