Issue #3433022: Separate class for container

Related to #3433022

src/Plugin/Field/FieldFormatter/IframeDefaultFormatter.php

@@ -191,7 +191,7 @@ class IframeDefaultFormatter extends FormatterBase {
     $options['id'] = $options['name'] = $htmlid;
 
     // Append active class.
-    $options['class'] = $item->class ?? '';
+    $options['class'] = $item->class ?? NULL;
 
     // Responsive start. Test class for iframe-responsive.
     $style = '#' . $htmlid . ' {' . $style . '}';