fix: treat an absent identifier as empty in CredentialSubjectProbe

[wantaekchoi]

A credentialSubject with neither id nor identifier currently validates as VALID. OB 3.0 §B.1.3 says "Either id or at least one identifier MUST be supplied", and the JSON Schema does not encode that constraint, so CredentialSubjectProbe is the only place it is enforced.

Cause

#46 swapped the inline array check for JsonNodeUtil.asNodeList, which returns null when the field is absent:

List<JsonNode> identifiers = JsonNodeUtil.asNodeList(root.get("identifier"));
if (identifiers == null || identifiers.size() > 0) return false;

A missing identifier takes the null branch and reports the subject as populated — the opposite of what the method's own comment says it checks ("Check that we have either .id or .identifier populated"). The check only fires today when identifier is written out as a literal [].

#46 was about accepting a non-array identifier, and asNodeList still does that. Only the null case moves, back to what it did before #46.

Change: identifiers == null || → identifiers != null &&

id	identifier	before	after
present	any	pass	pass
absent	array with entries	pass	pass
absent	single non-array node	pass	pass
absent	absent	pass	error
absent	[]	error	error

EndorsementInspector builds this probe through the one-argument constructor, which defaults identifierRequired to true, so endorsements see the same change. BitstringStatusListCredentialInspector passes false and is unaffected, and OB 2.0 does not use this probe.

Testing

Added simple-err-credential-subject-no-id-no-identifier.json, copied from the existing identifier-type fixture with identifier removed. There was no fixture for the absent case, which is why the suite did not catch this.

Before the fix, testSimpleJsonCredentialSubjectWithoutIdOrIdentifier fails with did not find probe id CredentialSubjectProbe; after it, the probe reports no id in credentialSubject. mvn -pl inspector-vc -am test passes, 98 tests.