Fix bug for PHP-FPM
Fix bug for PHP-FPM

PHP-FPM has a bug with STDIN: https://bugs.php.net/bug.php?id=73342

Running `clean-css-cli` in a PHP's `exec` will hit the bug, and PHP-FPM master's process will peak at 100% CPU usage.

Ignoring the `stdin` test if there's no argument can avoid the bug.
jakubpawlowicz/clean-css-cli