Thumbnail not exists
$meta_query = array( array( 'key' => '_thumbnail_id', 'compare' => 'NOT EXISTS', ), array( 'key' => '_supplier', 'value' => 'ewe', ), );
$meta_query = array( array( 'key' => '_thumbnail_id', 'compare' => 'NOT EXISTS', ), array( 'key' => '_supplier', 'value' => 'ewe', ), );