Check what extracted_pid is not empty

[atipugin]

If you don't set unicorn_pid directly in deploy.rb, fallback to unicorn_default_pid doesn't work (coz extracted_pid returns empty string and condition passes)

[junwchina]

I don't know why there is a unicorn -c #{config} to extract the pid file. As I know, all ruby codes run on local machine, the Capistrano just send the bash scripts to remote server to be execute. So i don't think it will work as our expected.