WSOD on "Add order items" modal with empty variation field

OrderItemsWidget::extractFormValues() doesn't check for empty #inline_form when $widget_state['form'] === 'add'.

Steps to reproduce from a clean install on PHP >8.4:

Scenario A:

1. Create a draft order and go to its view page.
2. Click "Add order items".
3. Click "Next" while the variation entity reference field is empty.
4. Click "Save".

Scenario B:

1. Create a product with a variation.
2. Create a draft order and go to its view page.
3. Click "Add order items".
4. Search for and select the created variation, click "Next", click "Add". The form receives the order items table showing the newly created order item.
5. Click "Add another" while the variation entity reference field is empty.
6. Click "Save".

Both scenarios result in WSOD. Observe a warning and a fatal error in the watchdog:

Warning: Undefined array key "#inline_form" in Drupal\commerce_order\Plugin\Field\FieldWidget\OrderItemsWidget->extractFormValues() (line 266 of [...]/commerce/modules/order/src/Plugin/Field/FieldWidget/OrderItemsWidget.php)

Error: Call to a member function getEntity() on null in Drupal\commerce_order\Plugin\Field\FieldWidget\OrderItemsWidget->extractFormValues() (line 268 of [...]/commerce/modules/order/src/Plugin/Field/FieldWidget/OrderItemsWidget.php).