Make VCDM credential subject a non-empty vec

[sbihel]

As described in https://www.w3.org/TR/vc-data-model/#credential-subject it is a MUST.

[timothee-haudebourg]

A verifiable credential MUST have a credentialSubject property.

In the context of JSON there's a difference between "the property must be there" and "the property must be non-empty". My interpretation of this sentence is "the property must be there" but implicitly it can be empty. Can we have a third opinion? Just to be sure 😅

[sbihel]

I don't have much of an opinion, it's just from https://w3c-ccg.github.io/vc-api-issuer-test-suite/#Issue%20Credential%20-%20Data%20Integrity