Question

How to exclude a post that has empty custom field (for example “slides”) from the wordpress loop?

Answer 1

You use meta_query in your Wp_query like this:

In this example they are only showing posts where the custom field “color” doesn’t have the value “blue”

$args = array(
'post_type' => 'product',
'meta_query' => array(
    array(
        'key' => 'color',
        'value' => 'blue',
        'compare' => 'NOT LIKE'
    )
)
);
 $query = new WP_Query( $args );

See this page: http://codex.wordpress.org/Class_Reference/WP_Query and scroll to the “Custom Field Parameters” section.