Fix invalid offset error.

media_attribution.module

@@ -107,10 +107,10 @@ function media_attribution_preprocess_entity_embed_container(&$variables) {
     $attribution = $attributions[0];
     $author = $attribution->get('field_attribution_author')->getValue()[0];
 
-    $source = $attribution->get('field_attribution_source')->getValue()[0];
+    $source = $attribution->get('field_attribution_source')->count() > 0 ? $attribution->get('field_attribution_source')->getValue()[0] : '';
     $license = $attribution->get('field_license')->entity;
     $license_link = !empty($license) ? $license->get('field_license_link')->getValue()[0] : FALSE;
-    $attribution_note = $attribution->get('field_attribution_note')->getValue()[0];
+    $attribution_note = $attribution->get('field_attribution_note')->count() > 0 ? $attribution->get('field_attribution_note')->getValue()[0]: '';
     $attribution_text = [];
     if (!empty($source['uri'])) {
       $attribution_text['source'] = [