feat(credentials): review pr feedbacks (#351)

Author: gabriel-peze

test/contracts/test_contract_referenced_credential.py

@@ -21,14 +21,14 @@ def test_defaults_match_platform_contract_expectations(self):
 
         self.assertEqual(field.key, ContractFieldKey.CredentialReference.value)
         self.assertEqual(field.label, "Select a credential reference")
-        self.assertTrue(field.mandatory)
+        self.assertFalse(field.mandatory)
         self.assertTrue(field.multiple)
         self.assertIsNone(field.credential_reference_type)
         self.assertEqual(field.type, ContractFieldType.CredentialReference.value)
         self.assertEqual(serialized["key"], "credential_reference")
         self.assertEqual(serialized["type"], "credential-reference")
         self.assertEqual(serialized["label"], "Select a credential reference")
-        self.assertTrue(serialized["mandatory"])
+        self.assertFalse(serialized["mandatory"])
         self.assertTrue(serialized["multiple"])
         self.assertIsNone(serialized["credential_reference_type"])