Disallow empty string for file_core.format

[theathorn]

For which schema is a change/update being suggested?

I would like to request an update to the file_core.json schema.

What should the change/update be?

Per DataBiosphere/azul#4097 there was a submission with an empty string for the required field "file_core.format". This caused a failure in the Azul indexer. An empty string should not be allowed for a required field as this circumvents the requirement to provide a valid value.

This update constitutes a minor change to the schema(s) it affects.

Why is the change requested?

Data contributors should be required to always provide a non-empty value for the required field file_core.format in order to prevent downstream processing and/or UI errors.