Show posts without term

I am working on a product page where I show reviews from my post type ‘reviews’, if the product page title matches with my term from the taxonomy ‘review-product’.

Example.

Product page title: Refrigerator

Show (reviews) posts with:

• Post type: ‘reviews’
• Taxonomy: ‘review-product’
• Term: Refrigerator

This works totally fine. But sometimes I have no reviews for a product (because there is no match between product title and term) and then I want to show ‘General’ reviews. In my website these are posts with:

• Post type: ‘reviews’
• Taxonomy: ‘review-product’
• Term: No term (!!!)

This posts don’t have a term from the taxonomy ‘review-product’.

My Question:

How can I show reviews posts without a term, if there are no reviews posts matching the product title.

This is what I have now:

function gtp_show_reviews( $number = 100, $term = null ) {
    $reviews = new WP_Query( array( 
        'post_type'         => 'reviews',
        'review-product'    => $term, // Fill this with product page slug (refrigerator)
    ));
    if( $reviews->have_posts() ) { 
        while( $reviews->have_posts() ) {
            $reviews->the_post();
            // The review
        }
    } 
    else {
        // Here I try to unset the 'review-product' term
        unset($reviews->query_vars['review-product']);
        unset($reviews->query_vars['term']);

        while( $reviews->have_posts() ) {
            $reviews->the_post();
            // Check if review has no term
            if( $reviews->has_term() == false ) {
               // The review
            }
        }   
    }

    wp_reset_postdata();
}

You cannot use the same object of WP_Query twice. Therefore you need to create another one with a tax_query parameter to fetch posts which are not assigned to any term.

//fetch all reviews which have no assigned term in 'review-product'
$taxonomy  = 'review-product';
$post_type="reviews";
$args = [
    'post_type' => $post_type,
    'tax_query' => [
        [
            'taxonomy' => $taxonomy,
            'terms'    => get_terms( $taxonomy, [ 'fields' => 'ids'  ] ),
            'operator' => 'NOT IN'
        ]
    ]
];

$query = new \WP_Query( $args );

The idea is to fetch a list of all terms of your taxonomy and pass them as argument to your tax-query with the NOT IN operator.

The second loop in your example should walk over the new WP_Query object.