Allow using untranslatable date fields

Currently we are looking for the value of the date field according to the language of the node but it is possible to have a multilingual node with an untranslatable date field, in which scenario the value of the field gets stored as und.

/**
 * Determine if the specific node is date-enabled.
 */
function _signup_date_get_node_scheduler($node) {
  $field = signup_date_field($node->type);
  if (!empty($node->{$field['field_name']}[$node->language][0]['value'])) {
    return 'date';
  }
  // Allow using an untranslatable date field.
  if (!empty($node->{$field['field_name']}['und'][0]['value'])) {
    return 'date';
  }
  if (isset($node->{$field['field_name'] . '_value'})) {
    return 'date';
  }
  return 'none';
}

As far as I know this logic is used multiple times in the module, so we should dig in and see which functions are affected. Possibly provide a helper function to get the value of the field.