Errors related to deleting buffer with ob_end_clean()

Warning message:

Notice: ob_end_clean(): failed to delete buffer. No buffer to delete in Drupal\bulk_content_operation\PHPExcelOperations::downloadTemplate() (line 60 of .../web/modules/contrib/bulk_content_operation/src/PHPExcelOperations.php)

This patches tests that there is content in the buffer before attempting the clean operation.