if ( !$value ) evaluates to TRUE when the value is 0.
Alternative:
if ( ! isset ( $_POST['_ordre'] ) )
return delete_post_meta( $post_id, '_ordre' );
update_post_meta( $post_id, '_ordre', $_POST['_ordre'] );
You don’t need add_post_meta(), the call to update_post_meta() will do that automatically.
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