Argument 2 passed to SearchApiAbstractService::configurationFormValidate() must be an array, null given

If a service has no configuration form, then form state will have no values for it (obviously).
While the submit handler has this code:

  $options = isset($values['options']['form']) ? $values['options']['form'] : array();
  $server->configurationFormSubmit($form['options']['form'], $options, $form_state);

the validate handler doesn't, and just passes form state directly:

$service->configurationFormValidate($form['options']['form'], $form_state['values']['options']['form'], $form_state);

which then triggers the fatal error when configurationFormValidate() gets NULL instead of an empty array.