docs: public Collection<MultifactorAuthenticationProvider> getAuthenticationProviderForService(final RegisteredService service) {
          val policy = service.getMultifactorPolicy();
          if (policy != null) {
              return policy.getMultifactorAuthenticationProviders().stream()
                  .map(MultifactorAuthenticationUtils::getMultifactorAuthenticationProviderFromApplicationContext)
                  .filter(Optional::isPresent)
                  .map(Optional::get)
                  .collect(Collectors.toSet());
          }
          return null;
      }