Loading missing/empty attributes

I understand that Null / NaN is not used, and it seems that for attributes there is no way to use an empty string. How do users generally signify an empty string? I could do something like ‘–Null–’ but just checking to make sure I’m not missing something. Do I need to use a flag column for any column that might be empty? Seems like if I use PyTigerGraph with an empty string “” I can upsert it, but in a file not. Any tips appreciated. Thanks.