Updated GetDeliveryServiceByXMLID function to return a DS nullable struct

CHANGELOG.md

@@ -90,6 +90,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added deep coverage zone routing percentage to the Traffic Portal dashboard.
 - Added a `traffic_ops/app/bin/osversions-convert.pl` script to convert the `osversions.cfg` file from Perl to JSON as part of the `/osversions` endpoint rewrite.
 - Added [Experimental] - Emulated Vault suppling a HTTP server mimicking RIAK behavior for usage as traffic-control vault.
+- Added Traffic Ops Client function that returns a Delivery Service Nullable Response when requesting for a Delivery Service by XMLID 
 
 ### Changed
 - Traffic Router:  TR will now allow steering DSs and steering target DSs to have RGB enabled. (fixes #3910)

traffic_ops/client/deliveryservice.go

@@ -58,6 +58,7 @@ func (to *Session) GetDeliveryServicesByServer(id int) ([]tc.DeliveryService, Re
 	return data.Response, reqInf, nil
 }
 
+// Deprecated: This function is deprecated and GetDeliveryServiceByXMLIDNullable will be used in the future.
 func (to *Session) GetDeliveryServiceByXMLID(XMLID string) ([]tc.DeliveryService, ReqInf, error) {
 	var data tc.GetDeliveryServiceResponse
 	reqInf, err := get(to, deliveryServicesByXMLID(XMLID), &data)
@@ -123,6 +124,16 @@ func (to *Session) GetDeliveryServiceNullable(id string) (*tc.DeliveryServiceNul
 	return &data.Response[0], reqInf, nil
 }
 
+func (to *Session) GetDeliveryServiceByXMLIDNullable(XMLID string) ([]tc.DeliveryServiceNullable, ReqInf, error) {
+	var data tc.DeliveryServicesNullableResponse
+	reqInf, err := get(to, deliveryServicesByXMLID(XMLID), &data)
+	if err != nil {
+		return nil, reqInf, err
+	}
+
+	return data.Response, reqInf, nil
+}
+
 // CreateDeliveryService creates the DeliveryService it's passed
 func (to *Session) CreateDeliveryService(ds *tc.DeliveryService) (*tc.CreateDeliveryServiceResponse, error) {
 	if ds.TypeID == 0 && ds.Type.String() != "" {