Extended check_subject_is_valid_test.py with a parametrized test (added "" country code)
Empty country code is now considered as a valid country code when creating a new certificate.
Changed tests accordingly.
Removed assert_invalid_subject_attribute_error function in check_subject_is_valid_test.py test
Improved check_subject_is_valid_test.py by using test parametrization.
Covered the new CertificateService check_subject_is_valid with unit tests.