I'm trying to use wp_nav_menu to only display a menu if one exists, otherwise, display nothing.

If I delete the menu, it will output a list of the pages.

Here are a couple of things I've unsuccessfully tried:

<?php            
function wp_get_nav_menu_object( $menu ) {
        if ( ! $menu )            
             wp_nav_menu( array( 'menu_class' => 'nav', 'container' => '' ) ); 
}
?>

I've also tried:

function wp_get_nav_menu_object( $menu ) {
    if ( ! $menu )
        return false;
    if ( $menu )
    return wp_nav_menu( array( 'menu_class' => 'nav', 'container' => '' ) ); 
}

How can I use wp_nav_menu to only display a menu if one exists, otherwise show nothing?