Add SKU to all products (for empty, TMP-{ID} will be used)

add_filter('the_content','aanpassing');
function aanpassing($content){
if ($_GET['start']=="ja"){
  echo 'start';
$query_args = array(
'post_type' => ['product', 'product_variation'],
  'posts_per_page' => -1,


);

// The Query
$the_query = new WP_Query( $query_args );

// The Loop
if ( $the_query->have_posts() ) {
	while ( $the_query->have_posts() ) {
		$the_query->the_post();
      	$sku = get_post_meta( get_the_ID(), '_sku', true );
      	if (empty($sku)){
          echo get_the_ID() .' '. get_the_title().' '. get_post_meta( get_the_ID(), '_sku', true ) .'<br>';
          update_post_meta(get_the_ID(),'_sku','TMP-'.get_the_ID());
        }
      
	}
	/* Restore original Post Data */
	wp_reset_postdata();
} else {
	// no posts found
}
}
  return $content;
}