The function wp_nav_menu takes an argument of fallback_cb which is the name of the function to run if the menu doesn’t exist.
so change you code to something like this:
function wp_nav_menu_no_ul()
{
$options = array(
'echo' => false,
'container' => false,
'theme_location' => 'primary',
'fallback_cb'=> 'fall_back_menu'
);
$menu = wp_nav_menu($options);
echo preg_replace(array(
'#^<ul[^>]*>#',
'#</ul>$#'
), '', $menu);
}
function fall_back_menu(){
return;
}
you can even remove the container from the menu and do other stuff with some more arguments sent to the wp_nav_menu function
Hope this helps.