Fix WooCommerce block's checkout error when opt-in is disabled

When opt-in is disabled it fails to validate checkout data, because it expects
mailpoet.optin to present. This PRs skips extending checkout data schema in case
the opt-in is disabled.
[MAILPOET-4004]

lib/PostEditorBlocks/WooCommerceBlocksIntegration.php

@@ -83,6 +83,9 @@ class WooCommerceBlocksIntegration {
   }
 
   public function extendRestApi() {
+    if (!$this->settings->get('woocommerce.optin_on_checkout.enabled', false)) {
+      return;
+    }
     $extend = Package::container()->get(ExtendRestApi::class);
     $extend->register_endpoint_data(
       [