Allow File::ALT_SEPARATOR to be nil when generating windows binary stub

Under Cygwin, I was repeatedly getting the following error when installing any gem with a `bin` subdirectory:

```
ERROR:  While executing gem ... (TypeError)
    no implicit conversion of nil into String
```

After examining the `gem install --backtrace ...` output, it appears to be due to `File::ALT_SEPARATOR` being `nil`.  This is a quick work-around for that issue.