fix: prevent insert error when creating deferred transaction by using…

… sane default value

src/oscar/apps/payment/abstract_models.py

@@ -137,7 +137,7 @@ def save(self, *args, **kwargs):
                 self._create_transaction(*txn)
 
     def create_deferred_transaction(
-        self, txn_type, amount, reference=None, status=None
+        self, txn_type, amount, reference="", status=""
     ):
         """
         Register the data for a transaction that can't be created yet due to FK