How to get latest post value if first post is empty in wordpress

Question

Your attempt was not to far away (+1 for trying, and crucially posting what you had tried), all you had to do really was add a compare of EXISTS to your meta query.

The meta_query parameter can be a bit of a beast to wrap your head around, and just in case you’ve not found it I’d recommend having a read of the Class Reference for WP_Query, specifically for Custom Field Parameters.

$args = array(
    'posts_per_page'    => 1,
    'paged'             => (get_query_var('paged')) ? get_query_var('paged') : 1,
    'order'             => 'DESC',
    'orderby'           => 'post_date',
    'meta_query'        => array(
        'relation'      => 'AND',
        array(
            'key'       => 'video',
            'compare'   => 'EXISTS'
        ),
        array(
            'key'       => 'video',
            'value'     => '',
            'compare'   => '!='
        )
    )

);
$loop = new WP_Query($args);

if($loop->have_posts()) : while($loop->have_posts()) : $loop->the_post();

        $specials = get_post_meta($post->ID, 'video'); 

        foreach($specials as $special): setup_postdata($special);

            echo '<iframe width="699" height="269" src="http://www.youtube.com/embed/'.$special.'?controls=1&showinfo=0&rel=0" allowfullscreen></iframe>';

        endforeach;

    endwhile;
    wp_reset_postdata();
endif;

Additional

As an added redundancy I’ve also added a check to ensure that the a video custom field actually contains a value. While I believe that the WP post edit screen will not actually allow you to save an empty custom value, I can’t be sure that you are updating it that way so thought it best to include.

However, if you don’t want to include that check you can change the meta_query parameter to this –

'meta_query'        => array(
    array(
        'key'       => 'video',
        'compare'   => 'EXISTS'
    )
)