Default missing data to null instead of 'UN'?

[bos666]

heron_encounter_style.sql defaults visit_dimension fields discharge_status and discharge_disposition 'UN'. Should these be defaulted to NULL or 'NI' instead per the CDM v3 spec with regard to "Missing or Unknown Data Values"?

[dckc]

NI is more appropriate than UN? I guess I don't know the difference. Here's hoping for time to look it up.

cc @njgraham