PSP Reference header fix (#184)

* Ensure that if PSP Reference is missing from response data or is empty, take the PSP Reference from the header instead

* Update client.py

Co-authored-by: Antoni Stroinski <55943882+antolo-arch@users.noreply.github.com>

Author: rutgervandriel

Adyen/client.py

@@ -632,9 +632,8 @@ def _handle_http_error(self, url, response_obj, status_code, psp_ref,
                 headers=headers, error_code=response_obj.get("errorCode"))
 
     @staticmethod
-    def _get_psp(response, header):
+    def _get_psp(response, headers):
         psp_ref = response.get('pspReference')
-        if psp_ref == "":
-            return header.get('pspReference')
-        else:
-            return psp_ref
+        if psp_ref is None:
+            psp_ref = headers.get('pspReference')
+        return psp_ref