images styles generation notice fix

web/modules/custom/images_styles_gen/src/CreateImagesStyles.php

@@ -8,7 +8,7 @@ namespace Drupal\images_styles_gen;
 class CreateImagesStyles {
 
   public static function createStyles($item, &$context){
-    if (!$context['sandbox']['progress']) {
+    if (!array_key_exists('progress', $context['sandbox'])) {
       $context['sandbox']['progress'] = 0;
     }
     $context['sandbox']['progress']++;