JSON null values don't validate

[smrgeoinfo]

apparently (with the Oxygen JSON schema validator), null is not a valid value for a field typed 'string'. so records (e.g. "registrant" and "samplingPurpose" in ark:/28722/k2028tb6n) don't validate.

I can change the data type in the JSON schema to ["string","null"], but I'm not sure if this will work from LinkML. Other missing values are populated with "Not Provided"; I'd recommend using that where ever there's a missing value, instead of Null. "" also validates.

see #80 for latitude and longitude values