:bug: adding feature images even if no related parent_id

plugins/cia-endpoints/includes/formats.php

     if($include_content) $filtered[content] = $item->post_content;
 
     $filtered[featured] = get_the_post_thumbnail_url($item);
-
+    if(!$filtered[featured]) {
+        $thumbnailId = get_post_meta( $item->ID, '_thumbnail_id', true );
+        $filtered[featured] = get_post($thumbnailId)->guid;
+    }
 
     // Materials + type endpoints
     // if($item->post_type === 'artist' || $item->post_type === 'event' || $item->post_type === 'exhibition') {