ENGCOM-3910: Order shipping method bug #20381

 - Merge Pull Request #20381 from maheshWebkul721/magento2:order-shipping-method-bug
 - Merged commits:
   1. 0759026
   2. 9c6af93
   3. bf246da
   4. 45c6fc7
   5. 82b1707
   6. ccd3014

Author: magento-engcom-team

app/code/Magento/Sales/Model/Order.php

@@ -1279,12 +1279,12 @@ public function getTrackingNumbers()
      * Retrieve shipping method
      *
      * @param bool $asObject return carrier code and shipping method data as object
-     * @return string|\Magento\Framework\DataObject
+     * @return string|null|\Magento\Framework\DataObject
      */
     public function getShippingMethod($asObject = false)
     {
         $shippingMethod = parent::getShippingMethod();
-        if (!$asObject) {
+        if (!$asObject || !$shippingMethod) {
             return $shippingMethod;
         } else {
             list($carrierCode, $method) = explode('_', $shippingMethod, 2);