Fix credential serializer when credentials model is None

src/api/src/backend/serializers/CredentialsSerializer.py

@@ -4,9 +4,10 @@
 class CredentialsSerializer:
     @staticmethod
     def serialize(model):
-
         credential = {}
-
+        if model == None:
+            return None
+
         credential["sk_id"] = model.sk_id
         credential["owner"] = model.owner
         credential["created_at"] = model.created_at