You can do the following:
echo '<div class="my_class">';
echo ($cart->count_product > 0) ? $cart->count_product : '';
echo '</div>';
If you want to have it inside your statement, do this:
if($cart->count_product > 0)
{
echo '<div class="my_class">'.$cart->count_product.'</div>';
}
You don't need the else
statement, since you're only going to output the above when it's truthy anyway.