Empty string field should result in null value

[henhuy]

This is a bit tricky, as JSON validation does not allow a type string to become null.
One workaround is to make field with two possible types (see json-editor/json-editor#649)
but this creates a strange dropdown.
Unfortunately, I think this must be handled using JS...

[jh-RLI]

if JSON validation does not allow string fields to be null it means that the null values are not used correctly as default for all fields in the oemetadata. What would be a better practice? simply "string": ""

We figured that null is always okay for unused JSON fields like this. If this is not true we can also consider specifying a different value for empty fields.