Fix return type of ftp_mlsd

[patrickrobrecht]

FtpClient::mlsd calls FtpWrapper::mlsd whichs calls ftp_mlsd which returns array|false (see manual for ftp_mlsd).

As the return type is in PHPDoc only, this does not affect runtime, but leads to wrong results within static analysis. For example, PHPStan reports an error in the following code suggested that the check is not necessary:

        $list = $this->ftp->mlsd($directoryName);
        /** @phpstan-ignore-next-line identical.alwaysFalse calls ftp_mlsd which returns false in case of errors */
        if ($list === false) {
            return [];
        }