Added simple structure to default graph

src/datasets.php

 <?php
 require_once(__DIR__ . "/ks-datasets-database.php");
 
-$EMPTY_SPACE = '{"links":[],"nodes":[],"objectGroups":[]}';
+$EMPTY_SPACE = '{"nodes":[{"id":0,"name":"Node","type":1},{"id":1,"name":"Node","type":1}],"links":[{"source":1,"target":0}],"objectGroups":[{"id":1,"name":"Default","color":"#000000"}]}';
 
 add_action("wp_ajax_get_space", "ks_get_space"); // Fires only for logged-in-users
 add_action("wp_ajax_nopriv_get_space", 'ks_get_space' ); // Fires for everyone