Checkboxes default values

Problem/Motivation

In Drupal 9.3.2 Drupal\Core\Render\Element\Checkboxes , it expects the '#default_value' to be an array or not present. In webform_validation_form_webform_ui_element_form_alter() they are set to NULL.

Steps to reproduce

Create a new webform and add a Advanced HTML component. The AJAX reports that there has been an error and the webform element does not appear.

Proposed resolution

Change the default from NULL to an empty array.

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None