fix static

Author: engcom-Charlie

app/code/Magento/Multishipping/Model/Checkout/Type/Multishipping.php

@@ -695,7 +695,7 @@ protected function _prepareOrder(\Magento\Quote\Model\Quote\Address $address)
         );
 
         $shippingMethodCode = $address->getShippingMethod();
-        if (isset($shippingMethodCode) && !empty($shippingMethodCode)) {
+        if ($shippingMethodCode) {
             $rate = $address->getShippingRateByCode($shippingMethodCode);
             $shippingPrice = $rate->getPrice();
         } else {