Set default product attribute when only one option

default_product_attribute.php

<?php
function get_variation_slug($taxonomy, $term_name)
{
  if (term_exists($term_name, $taxonomy)) {
    $term_slug = get_term_by('name', $term_name, $taxonomy)->slug;
  } else {
    $term_slug = '';
  }
  return $term_slug;
}

global $product;
if (!count($default_attributes = get_post_meta($product->get_id(), '_default_attributes'))) {
  $new_defaults = [];
  $product_attributes = $product->get_attributes();
  if (count($product_attributes)) {
    foreach ($product_attributes as $key => $attributes) {
      $values = explode(',', $product->get_attribute($key));
      if (count($values) == 1 && !isset($default_attributes[$key])) {
        $key_val = get_variation_slug($key, $values[0]);
        $new_defaults[$key] = $key_val;
      }
    }
    if(count($new_defaults)) {
      update_post_meta($product->get_id(), '_default_attributes', $new_defaults);
    }
  }
}