Allow `null` for optional parameters

This PR allows null for the optional parameters for the CBS API. Merging right away to cut a release so we can continue CashNet testing.

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

src/main/edu/wisc/services/cbs/order/header/OrderHeader.php

@@ -187,9 +187,9 @@ class OrderHeader extends MagentoOrder
     }
 
     /**
-     * @return string
+     * @return string|null
      */
-    public function getCustomerReferenceField(): string
+    public function getCustomerReferenceField()
     {
         return $this->customerReferenceField;
     }
@@ -198,7 +198,7 @@ class OrderHeader extends MagentoOrder
      * @param string $customerReferenceField
      * @return OrderHeader
      */
-    public function setCustomerReferenceField(string $customerReferenceField): OrderHeader
+    public function setCustomerReferenceField(string $customerReferenceField = null): OrderHeader
     {
         $this->customerReferenceField = $customerReferenceField;
         return $this;
@@ -223,9 +223,9 @@ class OrderHeader extends MagentoOrder
     }
 
     /**
-     * @return string
+     * @return string|null
      */
-    public function getInterfaceTypeFlag(): string
+    public function getInterfaceTypeFlag()
     {
         return $this->interfaceTypeFlag;
     }
@@ -234,16 +234,16 @@ class OrderHeader extends MagentoOrder
      * @param string $interfaceTypeFlag
      * @return OrderHeader
      */
-    public function setInterfaceTypeFlag(string $interfaceTypeFlag): OrderHeader
+    public function setInterfaceTypeFlag(string $interfaceTypeFlag = null): OrderHeader
     {
         $this->interfaceTypeFlag = $interfaceTypeFlag;
         return $this;
     }
 
     /**
-     * @return string
+     * @return string|null
      */
-    public function getDefaultSet(): string
+    public function getDefaultSet()
     {
         return $this->defaultSet;
     }
@@ -252,16 +252,16 @@ class OrderHeader extends MagentoOrder
      * @param string $defaultSet
      * @return OrderHeader
      */
-    public function setDefaultSet(string $defaultSet): OrderHeader
+    public function setDefaultSet(string $defaultSet = null): OrderHeader
     {
         $this->defaultSet = $defaultSet;
         return $this;
     }
 
     /**
-     * @return string
+     * @return string|null
      */
-    public function getTestMode(): string
+    public function getTestMode()
     {
         return $this->testMode;
     }
@@ -270,7 +270,7 @@ class OrderHeader extends MagentoOrder
      * @param string $testMode
      * @return OrderHeader
      */
-    public function setTestMode(string $testMode): OrderHeader
+    public function setTestMode(string $testMode = null): OrderHeader
     {
         $this->testMode = $testMode;
         return $this;