Settings Form - Warning: foreach() argument must be of type array|object, [#3590123]

Problem/Motivation

Warning caused by incorrect `#default_value` set on the `$form['prepublish']['group_types']['enabled_group_types']`.
Tested with Drupal 11.3.

Warning:

Warning: foreach() argument must be of type array|object, null given in Drupal\Core\Render\Element\Checkboxes::valueCallback() (line 119 of core/lib/Drupal/Core/Render/Element/Checkboxes.php).

Steps to reproduce

* Visit `/admin/config/system/siteimprove`

Comment	File	Size	Author
#3	siteimprove-3590123-MR46--20260514-1.diff	771 bytes	recrit

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3590123-fix-warning changes, plain diff MR !46
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