test(CredentialSubject): add null test

tests/v3_certificate_validation/test_unit_credential_subject.py

@@ -3,6 +3,24 @@
 from cert_issuer.models.verifiable_credential import validate_credential_subject
 
 class UnitValidationV3 (unittest.TestCase):
+    def test_null (self):
+        credential_subject = None
+
+        # TODO: ideally this would be stubbed to have better control over the test
+        # TODO: however urlretrieve and consumption is a bit convoluted to mock
+        credential_schema = {
+            "id": "https://www.blockcerts.org/samples/3.0/example-id-card-schema.json",
+            "type": "JsonSchema"
+        }
+
+        try:
+            validate_credential_subject(credential_subject, credential_schema)
+        except:
+            assert True
+            return
+
+        assert False
+
     def test_conforms_to_schema (self):
         credential_subject = {
             "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",