iki şekilde olur
<?php
$altmenu = $data->alt_menu1;

// 1. seçenek
if(isset($altmenu) && !empty($altmenu)){
echo "<ul class='dropdown-menu' role='menu'>
<li><a href='index-2.html'>".$data->alt_menu1."</a></li>
<li><a href='index-2.html'>".$data->alt_menu2."</a></li>
<li><a href='index-2.html'>".$data->alt_menu3."</a></li>
<li><a href='index-2.html'>".$data->alt_menu4."</a></li>
</ul>";
}

// 2. seçenek
$display = "none";
if(isset($altmenu) && !empty($altmenu)){
$display = "block";
}


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

?>