change condition query options, recovers f=null

Hi guys, thanks for the great job!
change condition query options, recovers f=null
#before

 // Because we can't get the desired url trough a url processor method
      // we iterate each result url and remove the facet params that haven't
      // been used on previous crumbs.
      foreach ($facet_results as $res) {
        $facet_url = $res->getUrl();
        /** @var \Drupal\Core\Url $facet_url */
        $query = $facet_url->getOption('query');
        $filter_key = $facet_source->getFilterKey() ?: 'f';
        $source_filter = $query[$filter_key];
        $source_filter = array_unique($source_filter);
        $source_filter = array_filter($source_filter);
        $matches = preg_grep('/^' . implode('|^', $facet_used_result) . '/', $source_filter);
        $query[$filter_key] = $matches;
        $facet_crumb_items[] = $res->getDisplayValue();
      }

#after

// Because we can't get the desired url trough a url processor method
      // we iterate each result url and remove the facet params that haven't
      // been used on previous crumbs.
      foreach ($facet_results as $res) {
        $facet_url = $res->getUrl();
        /** @var \Drupal\Core\Url $facet_url */
        $query = $facet_url->getOption('query');
        if($facet_source->getFilterKey()){
          $filter_key = $facet_source->getFilterKey();
          $source_filter = $query[$filter_key];
          $source_filter = array_unique($source_filter);
          $source_filter = array_filter($source_filter);
          $matches = preg_grep('/^' . implode('|^', $facet_used_result) . '/', $source_filter);
          $query[$filter_key] = $matches;
        }
        $facet_crumb_items[] = $res->getDisplayValue();
      }

if create custom url processor /admin/config/search/facets/facet-sources/search_api_you_facet_source/edit
and select facet value in breadcrumb /catalog/you_facet_alias/you_facet_value?f
Here is this line $filter_key = $facet_source->getFilterKey() ?: 'f';
Why do I need 'f' i'm not undestand
What do you think?
ty