Call to a member function getIsVirtual() on null

Magento 2.3.1, happens locally using PHP 7.2, after returned from my 3dsecure test gateway to confirm order success.

```
Fatal error: Uncaught Error: Call to a member function getIsVirtual() on null in /home/ccarnell/Sites/vax-uk2/vendor/magento/module-inventory-shipping/Observer/VirtualSourceDeductionProcessor.php:160 Stack trace: #0 /home/ccarnell/Sites/vax-uk2/vendor/magento/module-inventory-shipping/Observer/VirtualSourceDeductionProcessor.php(148): Magento\InventoryShipping\Observer\VirtualSourceDeductionProcessor->hasValidItems(Object(Magento\Sales\Model\Order\Invoice)) #1 /home/ccarnell/Sites/vax-uk2/vendor/magento/module-inventory-shipping/Observer/VirtualSourceDeductionProcessor.php(92): Magento\InventoryShipping\Observer\VirtualSourceDeductionProcessor->isValid(Object(Magento\Sales\Model\Order\Invoice)) #2 /home/ccarnell/Sites/vax-uk2/vendor/magento/framework/Event/Invoker/InvokerDefault.php(72): Magento\InventoryShipping\Observer\VirtualSourceDeductionProcessor->execute(Object(Magento\Framework\Event\Observer)) #3 /home/ccarnell/Sites/vax-uk2/vendor/magento/framework/Event/Invoker/InvokerDefault.php(60): Magento\Framework\Event\I in /home/ccarnell/Sites/vax-uk2/vendor/magento/module-inventory-shipping/Observer/VirtualSourceDeductionProcessor.php on line 160
```

Start with Magento\InventoryShipping\Observer\VirtualSourceDeductionProcessor.php, especially execute(), isValid(), and hasValidItems() around lines 92–160. Reproduce the 3D Secure return flow described in the issue and trace why the invoice reaches getIsVirtual() with a null value; done means the scenario no longer raises this fatal error and has coverage for the failing path.

Written by the indexing model from the issue text.