What is generally happening in these cases is that data is getting stringified, with `null` becoming the string "null" (which is a common output for string formatting a null in various languages).

This output is is then later re-parsed, and to try to restore null inputs back to being null, the code matches on the string "null", ignoring the fact that that could have been a valid string itself.

It's pretty believable to me that _some_ Java deserialization library from the early oughts totally does this as a default behavior.

Not all data structures are nullable or null reference is undesirable. In this case you can use the null object pattern https://en.wikipedia.org/wiki/Null_object_pattern#Descriptio...

I suspect some batch processing job with a CSV dump is the culprit behind a lot of bad treatment of strings.