Testing optional properties with null

[filip26]

Hi,
I'm not sure about this test input/credential-id-nonidentifier-fail.json.

It expects to fail when null on an optional property is set. Using null to test a required property is fine, but in a case of optional one, it might not be a good idea because many JSON and JSON-LD processors skip/omit properties with null. e.g. sometimes a special configuration is required to force a JSON parser to keep nulls.