fix(generate): Fix type error when rebuild query has no results

Closes #3528376

xmlsitemap.generate.inc

@@ -508,6 +508,11 @@ function xmlsitemap_rebuild_batch_fetch($entity, &$context) {
   $query->range(0, $limit);
 
   $result = $query->execute();
+  if (empty($result[$entity])) {
+    // If there are no more items to process we are done.
+    $context['finished'] = 1;
+    return;
+  }
   $ids = array_keys($result[$entity]);
 
   $info['xmlsitemap']['process callback']($ids);