Merge branch 'payment-null-pt-2' into 'master'

Remove `string` return type to allow null return values

See merge request !78

composer.json

 {
   "name": "adi-ia/cbs-techstore-client-php",
   "description": "A PHP client used to connect with the CBS API and perform requests",
-  "version": "7.1.1",
+  "version": "7.1.2",
   "license": "Apache-2.0",
   "homepage": "https://wiki.doit.wisc.edu/confluence/display/ADIIA/CBS",
   "authors": [

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

@@ -215,7 +215,7 @@ class OrderPayment extends MagentoOrder
     /**
      * @return string|null
      */
-    public function getPaymentNotes(): string
+    public function getPaymentNotes()
     {
         return $this->paymentNotes;
     }