[HPOS] Undefined index when taking a parent order out of trash

[a-danae]

Describe the bug

The following PHP notice appears in the logs when taking a parent order out of trash, and HPOS is enabled.

PHP Notice:  Undefined index: new in /var/www/html/wp-content/plugins/woocommerce-subscriptions-core/includes/class-wcs-object-data-cache-manager-many-to-one.php on line 36

To Reproduce

1. Go to WooCommerce -> Settings -> Advanced -> Features. /wp-admin/admin.php?page=wc-settings&tab=advanced&section=features
2. Under Experimental features, select "High performance order storage (new)", enable "Keep the posts and orders tables in sync (compatibility mode)", and save
3. As a shopper, purchase a subscription
4. As an admin, go to WooCommerce -> Orders
5. Move the Parent order for the subscription you just purchased to trash
6. Go to trash
7. Restore the Parent order you just moved to trash
8. Notice the PHP notice (ha!) in the debug.log file

PHP Notice:  Undefined index: new in /var/www/html/wp-content/plugins/woocommerce-subscriptions-core/includes/class-wcs-object-data-cache-manager-many-to-one.php on line 36

Expected behavior

Untrashing an order shouldn't trigger a PHP notice.

Actual behavior

Untrashing an order triggers a PHP notice.

Product impact

• [ ] Does this issue affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [ ] Does this issue affect WooCommerce Payments? yes/no/tbc, add issue ref

Additional context