How to do a query on custom taxonomies that is uncategorised?

I have a custom taxonomy and I wish to do a query for all custom posts who has no value assigned for that taxonomy (i.e. uncategorised).

This is what I have so far:

  $uncatQuery = new WP_Query( array(
                'post_type'         =>  'course',
                'tax_query' => array (
                    array(
                        'taxonomy' => 'course_category',
                        'field' => 'name',
                        'terms' => '',
                    )
                )
            ) );

However this does not work.

Please help!

NOTE: I manage to get it to work by creating a query to get all tax ids and then use a NOT IN operator, but that requires an additional query.

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Okay thanks for all the helpful comments. It turns out there is no way to do this using WP_QUERY alone.

The best way I can think of is to do it is like so:

$taxIds = get_terms($options['post-types'][$this->current_post_type->name], array(
                'fields'        => 'ids',
            ));
            $uncategorisedQuery = new WP_Query( array(
                'post_type'         =>  $postType,
                'posts_per_page'    => -1,
                'tax_query' => array (
                    array(
                        'taxonomy' => $taxonomyType,
                        'field' => 'term_id',
                        'terms' => $taxIds,
                        'operator' => 'NOT IN'
                    )
                )
            ) );

Although @shanbp had a nice idea of creating a “Uncategorised” taxonomy and automatically applying it to each post on creation if no taxonomy is selected.