One problem I see with the logic of using where Lead Owner is emtpy is when the lead is about to be assigned. In my case, I am running some validations before a lead is synched over. If the lead fails the validation I will assign it to a queue for other processing. If it passes I would like to call the assignment rules in SFDC. So this seem like a big error and should be fixed.
You can identify all leads who are owned by "a" queue by creating a list where SFDC Type = lead and Lead owner is empty.
Right now there is no out of the box way to create a list of leads owned by a specific queue(s), you would need to do something like create a custom field in SFDC to copy the queue name
You may wish to vote for this https://community.marketo.com/MarketoIdeaDetail?id=08750000000HRddAAG
Adchoices