Allow Payment Notes to be null

This PR allows null to be passed as a payment note. There might be more PRs like this as I find the lesser used CBS fields and make them less restrictive, but I'm going to merge these ASAP since they're just annotations and I'm doing some testing.

Notify: @ahoffmann @weizhong-wang @lloyd-carter @KJOYNER

src/main/edu/wisc/services/cbs/order/payment/OrderPayment.php

@@ -213,7 +213,7 @@ class OrderPayment extends MagentoOrder
     }
 
     /**
-     * @return string
+     * @return string|null
      */
     public function getPaymentNotes(): string
     {
@@ -221,10 +221,10 @@ class OrderPayment extends MagentoOrder
     }
 
     /**
-     * @param string $paymentNotes
+     * @param string|null $paymentNotes
      * @return OrderPayment
      */
-    public function setPaymentNotes(string $paymentNotes): OrderPayment
+    public function setPaymentNotes(string $paymentNotes = null): OrderPayment
     {
         $this->paymentNotes = $paymentNotes;
         return $this;