ShopperReference should be string to be aligned with checkout API (#2453

)

* ShopperReference should be string to be aligned with checkout API

Helper/PaymentMethods.php

@@ -266,9 +266,10 @@ protected function setQuote(\Magento\Quote\Model\Quote $quote): void
         $this->quote = $quote;
     }
 
-    protected function getCurrentShopperReference(): ?int
+    protected function getCurrentShopperReference(): ?string
     {
-        return $this->getQuote()->getCustomerId();
+        $customerId = $this->getQuote()->getCustomerId();
+        return $customerId ? (string)$customerId : null;
     }
 
     protected function getPaymentMethodsRequest(