Standardize management of empty attributes

[tloubrieu-jpl]

choose between null, nan or not having the attribute or other options

Acceptance criteria:
Check that specification is clear about the management of empty fields or attributes, see:

1. https://github.com/NASA-PDS/pds-api/blob/master/docs/spec/pds-api-specification.md
2. and online https://nasa-pds.github.io/pds-api/

[tloubrieu-jpl]

Added sentence in specification:
Missing values: If a property is optional or has an empty or null value, consider dropping the property from the JSON, unless there's a strong semantic reason for its existence (taken from https://softwareengineering.stackexchange.com/questions/285010/null-vs-missing-key-in-rest-api-response
)

See https://docs.google.com/document/d/16d0MVh48bFLvWsa5-B_Hy-cby1rGWdnNojWOJpUcOvA/edit?usp=sharing

Demo implementation updated to comply with that.