Remove unnecessary check in AddURL::addFieldValues()

In the "URL field" processor's addFieldValues() method, we currently have the following code:

      $fields = $this->getFieldsHelper()
        ->filterForPropertyPath($item->getFields(), NULL, 'search_api_url');
      foreach ($fields as $field) {
        if (!$field->getDatasourceId()) {
          $field->addValue($url->toString());
        }
      }

The if check in the foreach loop is unnecessary, as filterForPropertyPath() will already check for that (NULL there is treated as any other $datasource_id, not as "all values").