Method extractHeaders requires view to have data to generates the header row.

Hello,

I also needed to generate a CSV with headers even when there were no results.
I'm providing my code as an example for others who may have this issue.

In a *.module file, i declare a hook:

/**
 * Implements hook_views_post_render().
 */
function mymodule_views_post_render(ViewExecutable $view, array &$output, CachePluginBase $cache): void {
  if ($view->id() !== 'my_view_id') {
    return;
  }

  $options = $view->getStyle()->options;

  // The CSV output is empty when there is no result,
  // even if a header row is requested.
  if (
    empty($view->result) &&
    isset($options['csv_settings']['output_header']) &&
    $options['csv_settings']['output_header']
  ) {
    $header_row = [];
    foreach ($view->field as $name => $field) {
      $header_row[$name] = $field->label();
    }

    // Override output markup with the header row.
    $output['#markup'] = ViewsRenderPipelineMarkup::create(
      \Drupal::service('serializer')->serialize([$header_row],
        reset($options['formats']),
        ['csv_settings' => ['output_header' => FALSE] + $options['csv_settings']]
      )
    );
  }
}

Don't forget to rename the mymodule hook function name prefix with your module name and the my_view_id with your view ID.

Regards,