I'd guess some data transferred between systems as a homemade CSV. Empty field = empty string, but some field is nullable so someone decided that Null would be

I'd guess some data transferred between systems as a homemade CSV. Empty field = empty string, but some field is nullable so someone decided that Null would be a way to declare a null field. 20 years later and multiple systems depending on each others, random hidden CRONs in the middle and now people called Null have a problem.