thanks James but it doesn’t work – interestingly the following does work:
$args = array(
'post_type' => 'ze_knowledge',
'orderby' => 'post_date',
'order' => 'DESC',
'posts_per_page' => '-1',
'meta_key' => 'knowledge_video',
'meta_value' => ' ',
'meta_compare' => '!=',
);