only pass actual option strings to prevent ssh crashes

Using Net::OpenSSH it is currently possible to pass empty options to the ssh program which makes it crash every once in a while (at least on one of my machines).
Debugging on Net::OpenSSH level I found the following ssh command:
\# open_ex: ['ssh','-o','','-S', ...

Note the empty option string - isolated testing confirmed the same crashes with this.

So the changes in this commit make sure only non-zero options are passed along which avoids those types of crashes.

Cheers,
Simon