Ah, yes. I do have that module and have used it to filter by the custom date, but haven't discovered how to include the blank date entries as well. I might be missing something .
It is looking like I will have to use a placeholder date (maybe year 3000 or something ) and use logic in the item wrap code to display as empty if the date is that value. That would look cleaner at least on the frontend. Definitely not the best practice to hard-code it like that.
Maybe it would be possible to alter the Advanced List Filter module to include a NULL value filter as well...; I might take a look at the code and see how feasible that would be.