It is possible for date_augmenter_field_formatter_third_party_settings_form() to return NULL

Problem/Motivation

In Drupal CMS, if you try to disable Layout Builder for a content type -- any content type -- in the UI, you'll get an exception:

TypeError: Unsupported operand types: array + null in Drupal\field_ui\Form\EntityViewDisplayEditForm->Drupal\field_ui\Form\{closure}() (line 170 of core/modules/field_ui/src/Form/EntityViewDisplayEditForm.php). 

I traced this to date_augmenter_field_formatter_third_party_settings_form(), which can apparently return NULL in some situations. I am unfamiliar with the internals of this module so I'm not sure under what circumstances the NULL return can happen, but it's clearly possible.

Steps to reproduce

Install Drupal CMS, and then visit /admin/structure/types/manage/page/display and disable Layout Builder for that content type.

Proposed resolution

This function always has to return an array, so let's make sure it always does, and enforce that with a return type hint.