Fix statement in sanitizeFileName

[Prettycode-KC]

Seems like sanitizeFileName() fal_ftp/Classes/Driver/FTPDriver.php can not handle filenames like '0', which is likely to happen when typo3 creates _processed folders dynamically.

I assume this part causes the trouble:
if (!$cleanFileName) { throw new \TYPO3\CMS\Core\Resource\Exception\InvalidFileNameException( 'File name ' . $cleanFileName . ' is invalid.', 1320288991 ); }

[adgrafik]

Hi! Thanks for supporting!
The reason of "!" instead of FALSE ist that preg_replace don't return FALSE. I think
strlen($cleanFileName) === 0
should slove the problem. Can you fix this and try it again?